AUTOMATIC  TELEPHONY 


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AUTOMATIC  TELEPHONY 


A    COMPREHENSIVE    TREATISE   ON 
AUTOMATIC  AND  SEMI-AUTOMATIC  SYSTEMS 


AR 

MEMB 


SSEY  SMITH,  E.E. 

.,   FORMERLY   PROFESSOR   OF   TELEPHONE 
rNEERING,    PURDUE   UNIVERSITY 


AND 


WILSON  LEE  CAMPBELL,  E.E. 

FELLOW   A.    I.    E.    E. 


Second    Edition 


McGRAW-HILL  BOOK  COMPANY,  Inc. 

NEW  YORK:    370   SEVENTH   AVENUE 

LONDON:    6  &  8  BOUVERIE  ST.,  E.  C.  4 

1921 


Copyright,  1914,  1921,  by  the 
McGraw-Hill  Book  Company,  Inc. 


THK     JIAH.K     PKTCSS     V  o  B  K     PA. 


fogmeeriiig 
Lbctiy 

TK 


Oo 


3 


C5  MR.  ALEXANDER  ELLSWORTH  KEITH 

J__^  WHO   BY   HIS   GREAT    RESOURCEFULNESS,    UNLIMITED   ENTHUSIASM, 

h3  STEADY   DETERMINATION   AND   KEEN   FORESIGHT 

£tj  HAS  BEEN   FOR  TWENTY-SEVEN   YEARS   THE   LEADER  IN   THE 

S3  DEVELOPMENT  OF   AUTOMATIC   TELEPHONY, 

ry*  THIS  VOLUME   IS   RESPECTFULLY   DEDICATED. 


cs 


48S231 


PREFACE  TO  SECOND  EDITION 

Prior  to  the  year  1908,  the  use  of  automatic  and  semi-automatic 
telephone  apparatus  was  confined  almost  entirely  to  the  "Independent" 
telephone  companies  of  the  United  States  of  America. 

Since  that  year,  its  adoption,  first,  by  the  Governments  of  various 
Canadian  cities  and  provinces,  then  by  the  Governments  of  Germany, 
Austria  and  France,  and,  latterly,  by  the  Governments  of  England, 
Italy,  Australia  and  New  Zealand;  and  its  introduction  into  Denmark,  the 
Argentine  Republic,  Cuba,  the  Hawaiian  Islands  and  the  lands  of  the 
far  East,  has  created  a  world-wide  interest  in  this  system  and  a  general 
demand  among  telephone  engineers  for  a  more  extensive  knowledge  of  it. 

An  endeavor  to  meet  this  demand  has  resulted  in  the  preparation  of 
this  volume,  which  is  the  first  to  be  devoted  exclusively  to  automatic  and 
semi-automatic  telephony,  or  which  has  attempted  to  treat  the  art  in  the 
manner  which  its  importance  requires. 

Even  within  the  pages  of  this  book  the  authors  have  found  it  impracti- 
cable to  narrate  the  full  details  of  the  practice  of  all  the  manufacturers. 
They  therefore  have  described  fully,  typical  circuits  and  apparatus  of 
the  Strowger  type,  with  brief  outlines  of  other  important  systems.  The 
authors  feel  that  the  principles  and  methods  brought  out  in  these  chapters 
are  sufficiently  applicable  to  other  makes  of  equipment  to  supply  the 
wants  of  students  of  the  general  art. 

Radical  changes  have  been  made  in  revising  this  book  from  the  first 
edition.  As  far  as  possible,  only  live  matter  has  been  retained,  live 
subjects  introduced,  and  old  things  re-cast  into  such  form  as  will  be  more 
readily  understood.  The  writers  do  not  believe  in  coddling  those  who 
are  too  lazy  to  work.  Much  of  the  thorough  knowledge  which  people 
need  is  to  be  obtained  only  by  patient,  persistent,  consistent  study. 
But  there  is  no  need  of  hiding  those  fundamental  principles  which  are 
simple  behind  the  customary  mass  of  physical  material  which  is  used  in 
putting  those  principles  into  practice.  For  this  reason  most  of  the 
electrical  diagrams  have  been  rearranged  from  the  stand-point  of  the 
requirements  of  teaching. 

The  authors  wish  to  express  their  thanks  to  Mr.  Fred  L.  Baer,  of 
Chicago,  for  his  assistance  on  traffic  and  trunking,  to  Mr.  P.  V.  Christen- 
sen,  of  Copenhagen,  Denmark,  for  information  concerning  his  traffic 
distributer  system,  to  Mr.  E.  L.  Grauel,  of  Rochester,  New  York,  for 
information  on  the  practical  uses  of  the  private  automatic  exchange,  to 
Dr.  Frank  B.  Jewett,  chief  engineer  of  the  Western  Electric  Company, 
New  York  City,  for  information  concerning  the  system  of  that  company, 


viii  PREFACE  TO  SECOND  EDITION 

and  to  Mr.  Harry  E.  Hershey,  of  Chicago,  for  his  assistance  in  the 
preparation  of  technical  matter  and  copy. 

They  also  wish  to  acknowledge  the  assistance  and  co-operation  of  a 
number  of  the  members  of  the  engineering  staff  of  the  Automatic  Electric 
Company,  and  thus  publicly  to  express  to  them  their  appreciation  and 
thanks. 

The  Authors. 
Chicago,  III., 

January,  1921. 


PREFACE  TO  FIRST  EDITION 

Prior  to  the  year  1908,  the  use  of  automatic  and  semi-automatic 
telephone  apparatus  was  confined  almost  entirely  to  the  "Independent" 
telephone  companies  of  the  United  States  of  America. 

Since  that  year,  its  adoption,  first,  by  the  Governments  of  various 
Canadian  cities  and  provinces,  then  by  the  Governments  of  Germany, 
Austria  and  France,  and,  latterly,  by  the  Governments  of  England, 
Italy,  Australia  and  New  Zealand;  and  its  introduction  into  Denmark, 
the  Argentine  Republic,  Cuba,  the  Hawaiian  Islands  and  the  lands  of  the 
far  East,  has  created  a  world-wide  interest  in  this  system  and  a  general 
demand  among  telephone  engineers  for  a  more  extensive  knowledge  of  it. 

An  endeavor  to  meet  this  demand  has  resulted  in  the  preparation  of 
this  volume,  which  is  the  first  to  be  devoted  exclusively  to  automatic  and 
semi-automatic  telephony,  or  which  has  attempted  to  treat  the  art  in  the 
manner  which  its  importance  requires. 

Even  within  the  pages  of  this  book  the  authors  have  found  it  impracti- 
cable to  narrate  the  full  details  of  the  practice  of  all  the  manufacturers. 
They  therefore  have  described  fully,  typical  circuits  and  apparatus  of 
each  of  the  more  important  or  instructive  types  on  the  market,  but  have 
found  it  necessary  to  confine  their  discussion  of  such  subjects  as  "  Traffic," 
"Development  Studies,"  "Central  Office  Building  Design,"  "Long 
Distance  Line  Equipment,"  etc.,  to  the  practice  of  some  one  manufacturer 
Wherever  this  has  been  found  necessary,  the  practice  of  the  Automatic 
Electric  Company  has  been  followed,  and  the  authors  feel  that  the 
principles  and  methods  brought  out  in  these  chapters  are  sufficiently 
applicable  to  other  makes  of  equipment  to  supply  the  wants  of  students  of 
the  general  art. 

The  authors  wish  to  express  their  thanks  to  Mr.  P.  V.  Christensen,  of 
Copenhagen,  Denmark,  for  information  concerning  his  traffic  distributer 
system;  to  Mr.  David  S.  Hulfish,  of  the  Canadian  Machine  Telephone 
Company,  Brantford,  Canada,  for  information  concerning  the  Lorimer 
system;  to  Dr.  A.  Rapp,  Berlin,  Germany,  for  information  on  the  system 
of  the  Siemens-Halske  Company;  and  to  Mr.  Gerard  Swope,  New  York, 
for  his  assistance  in  the  preparation  of  the  description  of  the  system  of  the 
Western  Electric  Company.  They  also  wish  to  acknowledge  the  assist- 
ance and  co-operation  of  a  number  of  the  members  of  the  engineering 
staff  of  the  Automatic  Electric  Company,  and  thus  publicly  to  express 
to  them  their  appreciation  and  thanks. 

The  Authors. 
Chicago,  III., 
October,  1914. 

ix 


CONTENTS 

Page 

1.  Introduction 1 

2.  Trunking 3 

3.  Automatic  Electric  Company's  Apparatus 38 

4.  Trunking,  Its  Physical  Arrangements  and  Variations 107 

5.  Subscriber's  Station  Equipment,  Automatic  Electric  Company      .    .    .  151 

6.  Private  Automatic  Exchange 164 

7.  Measured  Service  Equipment 192 

8.  Automatic  Traffic  Distributer  Equipment 204 

9.  Automatic  Sub-offices  in  Connection  with  Manual  Central  Offices  226 

10.  The  System  of  the  Western  Electric  Company 236 

11.  Automanual  System 253 

12.  Long  Distance  and  Suburban  Equipment 277 

13.  Rural  Automatic  Telephones 309 

14.  Cut-overs  and  Interconnections  of  Manual  and  Automatic  Offices   .  325 

15.  Power  Plant,  Supervisory  and  Testing  Equipment 345 

16.  Traffic 381 

17.  Development  Studies 404 

Index 425 


AUTOMATIC  TELEPHONY 


CHAPTER  I 
INTRODUCTION 

The  telephone  was  invented  in  1876  and  exhibited  at  the  Centennial 
Exhibition  at  Philadelphia. 

The  first  automatic  telephone  switching  apparatus,  that  of  Connolly 
and  McTighe,  was  invented  not  long  after  and  a  patent  applied  for  in 
1879.  Thus  the  idea  of  remote  controlled  electrical  switches  for  con- 
necting telephone  lines  is  almost  as  old  as  the  telephone  itself. 

The  Strowger  automatic  system  originated  about  1887,  and  its  first 
patent  was  applied  for  in  1889. 

The  commercial  development  began  with  the  La  Porte,  Indiana, 
exchange  in  1893,  and  has  continued  steadily  ever  since. 

The  conditions  of  labor  became  more  difficult  with  every  passing 
year.  The  world  war  rendered  the  situation  acute.  These  conditions 
led  the  largest  operating  companies  in  America  publicly  to  adopt  the 
automatic  on  a  comprehensive  scale.  Foreign  governments  also  started 
similar  measures  of  relief  for  a  condition  which  was  almost  intolerable  to 
the  public. 

Today  (1921)  the  automatic  switching  of  telephone  lines  is  the  fore- 
most subject  in  telephony. 

"Automatic"  telephony  is  the  term  applied  to  the  use  of  machinery 
to  perform  those  parts  of  the  work  of  switching  which  were  formerly 
done  by  hand.  See  definitions  of  "automatic"  in  Webster's  New  Inter- 
national Dictionary  (1915),  Standard  Dictionary  (1913),  and  Worcester's 
Dictionary  (1897). 

The  requirements  of  a  telephone  subscriber  must  be  stated  in  terms 
which  are  independent  of  apparatus  and  methods.  Much  error  will  be 
avoided  if  we  divest  our  minds  of  the  conditions  imposed  by  any  one 
means  of  rendering  service. 

The  subscriber  requires,  without  unnecessary  delay,  without  undue 
stress  on  himself,  to  be  connected  to  the  telephone  which  he  desires  (and 
to  no  other),  to  talk  to  the  called  subscriber  with  ease  and  without  inter- 
ruption or  eavesdropping,  and  to  have  this  service  available  continuously. 
Stated  concisely,  the  requirements  are: 

1 


2  AUTOMATIC  TELEPHONY 

1.  Speed  of  connection  and  disconnection. 

2.  Ease  of  obtaining  connection  and  disconnection. 

3.  Accuracy  of  connection. 

4.  Voice  transmission. 

5.  Secrecy. 

6.  Continuous  service. 

The    manual    switching   of    telephone    lines    involves    the    following 
inherent    features: 

1.  One    or   more    human    intermediaries    between    the    subscribers. 
This  leads  to: 

2.  Translation  of  calling  subscriber's  desires  from  sound  into  ideas; 

3.  Translation  of  these  ideas  into  action; 

4.  Common  spoken  language  between  subscriber  and  operator; 

5.  Crowding  many  operators  together  in  a  place  difficult  to  ventilate; 

6.  Fluctuations  in  service  due  to  personality  and  health  of  operator 
and  the  amount  of  traffic; 

7.  Immediate  impairment  of  service  by  strikes,  epidemics, etc.; 

8.  Co-operation  of  operator  with  subscriber  in  securing  his  desires; 
largely  negatived  by  the  machine-like  conditions  of  the  work. 

Automatic  switching  involves  the  following  inherent  features: 

1.  Nothing  but  automatic  switches  between  the  subscribers.     This 
leads  to: 

2.  The  definite  control  of  the  automatic  switches  by  the  subscriber 
(by  electrical  impulses) ; 

3.  The  accurate  response  of  the  switches  to  the  control,  over  lines  of 
varying  properties; 

4.  Uniform  service,  almost  independent  of  traffic,  public  health,  and 
personnel; 

5.  Absence  of  human  co-operation,  except  when  desired  and  requested. 
Automatic  telephony  may  be  divided  for  study  into  (1)  trunking*,  (2) 

mechanism,  and  (3)  electrical  circuits.     These  will  be  treated  separately, 
with  such  cross-references  as  are  helpful. 


CHAPTER  II 
TRUNKING 

The  methods  of  trunking  which  are  about  to  be  described  are  pecu- 
liarly adapted  to  automatic  switching,  and  are  among  the  chief  factors 
in  the  success  of  automatic  systems.  The  general  plan  of  an  automatic 
exchange  divides  the  subscribers'  lines  into  groups  and  sub-groups.  A 
10,000-line  exchange  is  composed  of  ten  groups,  each  having  1000  lines. 
Generally  each  1000-line  group  is  subdivided  into  ten  smaller  groups  of 
100  lines  each.  Two  variations  from  this  plan  should  be  noted,  namely, 
a  final  subdivision  into  50-line  groups  (practised  by  the  late  American 
Automatic  Telephone  Company)  and  into  200-line  groups  (installed  by 
the  Western  Electric  Company  in  Europe).  These  variations  will  be 
referred  to  more  fully  later  and  the  present  discussion  will  be  confined 
to  the  plan  more  generally  used.  For  the  purpose  of  selecting  the  line 
of  any  subscriber  the  100  lines  of  any  unit  are  usually  considered  as  made 
up  of  ten  groups  each  having  ten  lines. 

The  groups  and  sub-groups  need  not  follow  the  decimal  system,  but  it 
is  more  convenient  to  have  them  do  so,  since  the  numbers  in  common  use 
are  thus  expressed  and  may  be  used  as  call  numbers  without  change. 

When  the  decimal  system  of  grouping  is  used,  the  digits  of  the  call 
number  indicate  the  location  of  the  line  to  which  it  belongs.  Thus,  No. 
4375  indicates  line  No.  75  in  the  third  hundred  of  the  fourth  thousand. 

The  selection  of  a  line  proceeds  by  successive  choice.  First  the  thou- 
sand is  selected,  then  a  certain  hundred  in  that  thousand,  and  finally  the 
ten  and  unit  in  the  selected  hundred. 

We  will  first  describe  a  100-line  system  and  build  it  up  to  the  larger 
sizes. 

One-hundred-line  System. — One  hundred  telephones  may  be  given 
service  by  equipping  each  line  with  a  connector  switch,  which  has  the 
ability,  under  the  control  of  the  subscriber,  to  connect  with  any  of  the 
hundred  lines.  In  Fig.  1,  let  the  calling  telephone  be  connected  by  two 
wires  to  a  pair  of  springs  or  wipers.  Associated  with  these  wipers  is  a 
bank  of  contacts,  each  of  which  is  a  small  flat  brass  piece.  They  are 
arranged  in  pairs  (100  pairs)  one  for  each  subscriber's  line.  The  sub- 
scriber at  the  calling  telephone  can  move  the  line  wipers  of  his  connector 
switch  so  that  they  will  come  into  contact  with  any  pair  of  the  hundred. 

The  numbering  of  each  pair  of  the  bank  contacts  is  based  upon  the 
number  of  steps  required  to  lift  and  rotate  the  wipers  to  them.  It 
requires  only  one  step  to  lift  the  wipers  to  the  first  or  lowest  level,  hence 

3 


4  AUTOMATIC  TELEPHONY 

all  numbers  in  that  level  begin  with  "1."  After  having  been  lifted  to 
the  first  level,  four  steps  are  required  to  rotate  the  wipers  to  contact  4, 
so  that  this  will  be  termed  "14."  Similarly,  to  connect  with  telephone 
47  requires  four  vertical  steps  and  seven  rotary  steps. 

02         03         04  05         06         07         08         09 


PI  E^Ti 
© 

o 


II  12  13 


Calling 
Telephone 


*l 


.  90 

!ao 
!?o 
>o 
>o 

>0 
'20 


Ife  17  18  19  10 


o 


o 


No.  14  No.  47 

Fig.   1. — Essential  principle  of  connector  switch. 


The  order  of  numbers  shown  for  the  bottom  level  is  characteristic  of 
all  the  levels.  It  starts  with  "11"  and  runs  in  regular  order  to  "19," 
which  is  followed,  not  by  "20"  but  by  "10."  This  is  because  the  digit 
"0"  means  "ten,"  so  that  the  number  "10"  signifies  one  vertical  step 
and  ten  rotary  steps. 


Normal 
L  ines  ■'. 


Telephones 


M 


93 


Line 
Wipers 
'Lines 


Connecto:'  I  witches 


Bank  Multiple : 


Line 
Wipers 


Line 
Wipers 


No.  00       * 

Connector 


Fig.   2. — Principle  of  100-line  exchange. 

A  complete  100-line  automatic  exchange  requires  only  100  connector 
switches,  interconnected  so  that  any  subscriber  can  extend  his  line  to 
that  of  any  other  subscriber.     (Fig.  2.)     Three  telephones  are  here  used 


T  RUN  KING  5 

to  show  the  relations  of  the  entire  hundred.  Imagine  that  the  single  line 
running  from  each  telephone  to  its  line  wipers  consists  of  two  wires,  and 
that  each  little  circle  in  the  connector  bank  represents  two  or  more  con- 
tacts. Each  telephone  has  its  own  connector  switch,  the  wipers  of  which 
are  represented  by  an  arrow. 

Besides  being  connected  to  the  line  wipers  of  its  own  connector,  each 
line  is  multiplied  to  a  contact  pair  in  every  bank.     For  example,  the 


Symbol 

Condensed     —————————— 

Symbol  —————————— 

•— >  zzzzzzzzzz 

o  .      C»  '  "Bank" 

Wiper 

Fig.  3. — Symbol  of  major  switch. 


Condensed 
Symbol 

□ 


line  from  telephone  No.  14  runs  to  the  wipers  of  connector  No.  14,  and 
also  to  contact  pair  No.  14  in  every  connector  switch  in  the  hundred. 
Those  wires  which  join  the  bank  contacts  together  are  termed  the  "bank 
wiring,"  and  those  wires  which  connect  the  bank  multiple  to  the 
subscribers'  lines  are  called  "normal  lines." 

Upon  the  connector  switch  devolve  certain  duties,  among  which  may 
be  mentioned  the  control  of  the  wipers  by  the  subscriber,  a  busy  test, 
means  for  ringing,  and  means  for  releasing.  If  the  system  be  designed 
for  common  battery  talking,  the  connector  must  furnish  direct  current 
to  the  lines. 


SYMBOL 


w 


One  Leve/ 


^jjpujjjj^u 


Incoming  J ; 
Trunks     l- 


4  Etc.-hol05w. 
Inclusive 


''•Jacks 


Fig.  4. — Connectors  multipled. 


Symbols. — From  time  to  time  symbols  peculiar  to  automatic  tele- 
phony will  be  introduced.  The  connector-switch  symbol  (Fig.  1)  is 
reduced  to  the  two  symbols  shown  in  Fig.  3.  The  multipled  connectors 
of  Fig.  2  are  reduced  to  the  symbol  of  Fig.  4. 


6  AUTOMATIC  TELEPHONY 

A  "major  switch"  is  one  which  performs  a  major  act  in  switching. 
A  "minor  switch"  is  one  which  has  purely  auxiliary  functions. 

Non-numerical  Switches. — When  each  subscriber  has  one  connector, 
there  is  considerable  waste  of  apparatus,  because  not  very  many  of  them 
are  in  use  at  the  same  time. 

If  a  residence  telephone  originates  ten  calls  per  day  (each  lasting  two 
minutes)  and  receives  the  same  number  of  calls,  these  20  calls  occupy  the 
telephone  40  minutes  per  day,  which  is  a  liberal  allowance  for  this  class 
of  service.  This  is  only  2.8  per  cent  of  the  day.  A  calculation  for  a 
business  telephone,  allowing  40  calls  (20  in  and  20  out)  at  one  and  one- 
half  minutes  each,  shows  an  actual  occupancy  of  the  apparatus  of  scarcely 
more  than  4  per  cent  of  the  time.  Yet  the  telephone  company  must 
maintain  all  the  apparatus  which  is  individual  to  the  line  and  pay  interest 
and  depreciation  on  it  24  hours  per  day  in  order  that  it  may  be  used  3  or 
4  per  cent  of  the  time. 

It  is  therefore  standard  practice  to  interpose  non-numerical  switches 
between  the  subscriber  lines  and  a  reduced  number  of  connectors  (usually 
ten  per  hundred  lines).  A  non-numerical  switch  is  one  which  will 
enable  the  subscriber  (without  his  knowledge)  to  get  the  use  of  an  idle 
connector  switch. 

Lineswitches. — A  lineswitch  is  a  non-numerical  switch,  attached  to 
the  subscriber  line,  which  can  connect  the  line  to  a  trunk  leading  to  an 

idle  connector  (or  other  switch  in  the  larger 

systems) . 

0  A  rotary  lineswitch  (Fig.  5)  is  one  whose 

o        subscriber  line  comes  to  the  wipers  and  whose 

bank  contains  the  trunk  terminals.    It  usually 

Fig.  5. — Symbol  of  rotary  1  ,  •         /•  i  \ 

lineswitch  nas  one  m°t10n  (in  one  plane). 

A  Keith  lineswitch,  plunger  type,  Fig.  6, 
accomplishes  the  same  result  with  both  the  subscriber  line  terminals  and 
the  trunk  terminals  built  into  a  bank,  and  employs  a  moveable  plunger 
to  cause  the  connection.  There  is  a  master  switch  to  select  idle  trunks 
for  each  group  of  lineswitches.     (See  page  45.) 

Finder  Switches. — A  finder  switch,  Fig.  7,  is  a  non-numerical  switch, 
attached  to  a  trunk  leading  to  a  connector  (or  other  switch  in  the  larger 
systems).  On  its  banks  are  subscriber  lines,  multipled  also  to  other 
finder  banks  in  the  same  group.  The  finder  of  an  idle  connector  can 
move  its  wipers  into  contact  with  a  calling  line.  It  may  have  one  or 
two  motions,  rotary  only,  or  vertical  and  rotary. 

One-hundred-line  System. — The  effect  of  lineswitches  on  a  100-line 
system  of  lineswitches  of  either  type  is  to  reduce  the  connectors  from  one 
hundred  to  an  average  of  ten.  Figure  8  shows  such  an  exchange  with 
rotary  lineswitches.  Each  subscriber  line  is  multipled  to  its  own  set  of 
contacts  on  the  bank  of  each  connector;  it  also  runs  to  the  wipers  of  its 


/■ 


T  RUN  KING  7 

own  rotary  lineswitch.  The  individual  apparatus  in  the  central  office 
consists  of  a  lineswitch  and  a  set  of  contacts  on  each  connector  bank. 
The  connector  mechanisms  and  wipers  are  common  to  all — they  may  be 
used  by  any  subscriber. 

Line?  Swfhchos> 
—o £-^> 


^Normal 
C&b/e 

Fig.   6. — Symbol  of  plunger  lineswitch. 


_J 


Line 

Finder 


V 


Trunk 


V 


Trunk 


■V 


Trunk 


Fig.   7. — Symbol  of  finder  switch. 

The  normal  cable,  starting  from  the  multipled  banks  of  the  connectors, 
runs  to  the  plunger  side  of  the  lineswitches,  each  line  being  attached  at 
that  point  to  the  subscriber's  line  to  which  it  belongs.  This  cable  may  be 
very  short,  for  the  connectors  are  often  mounted  on  the  back  side  of  the 
same  frame  which  carries  the  lineswitches. 

When  a  subscriber  takes  his  receiver  from  the  hook,  his  lineswitch 
operates  and  connects  his  line  to  a  trunk  and  therefore  to  a  connector 


8 


A  U  TO  MA  TI C  TELEPHON  Y 


switch.  The  subscriber  then  operates  his  dial,  lifting  and  rotating  the 
wipers  of  the  connector  to  the  desired  contacts.  The  connection  now 
extends  through  the  normal  cable  to  the  lineswitch  of  the  called  line 
and  thence  to  the  called  telephone.  The  lineswitch  of  the  called  tele- 
phone is  merely  a  meeting  point :  the  line  does  not  go  through  it.  The 
same  exchange  equipped  with  Keith  lineswitches  is  shown  in  Fig.  9. 


lOOTelephones 


Line  Switches 


Fig.   8. — One-hundred-line  system  with  rotary  lineswitches. 

Thousand-line  System. — The  1000-line  system  is  built  up  out  of  ten 
100-line  groups.  The  selection  is  based  on  first  choosing  the  unit  or 
group  in  which  the  called  line  is  located  and  then  picking  out  the  par- 
ticular line  in  that  group.     (See  Fig.  10.) 

The  telephone  lines  come  into  the  central  office  to  lineswitch  wipers 
and   connector  banks   as   before.     The  lineswitch   banks  have   trunks 


8                        Line  Switches 

'^J  Wiper  or Plunger 

Bank 

Connectors 

=                                            t 

I 

r-""~ 

•d 

— 

^ 

£ 

^m 

vj 

■^\ 

I 

■5 

i > 

s 

"""^ 

"™ 

*^ 

§ 

^ 

Normal  Cable 

h 

Fig.  9. — One-hundred-line  system  with  Keith  lineswitches. 

running  to  the  wipers  of  selectors.     The  selector  banks  are  multipled 
together  throughout  the  exchange. 

The  connector  wipers  receive  trunks  from  selector  bank  levels  accord- 
ing to  groups.  All  the  trunks  from  the  first  level  run  to  connectors  in 
the  first  hundred,  from  the  second  level  to  the  second  hundred,  etc. 


T RUN KING 


9 


Though  only  the  first,  second,  and  eighth  hundreds  are  represented,  the 
others  are  easily  imagined. 

The  first  function  of  the  selector  (to  select  the  level)  must  be  com- 
pletely under  the  control  of  the  subscriber.  The  second  (to  hunt  an 
idle  trunk)  should  be  completely  self-acting,  for  it  is  of  no  interest  to  the 
subscriber  which  trunk  or  connector  he  gets. 

Let  us  suppose  that  subscriber  No.  261  desires  a  connection  with 
telephone  No.  805.  The  calling  subscriber  first  removes  his  receiver 
from  the  hook.     This  enables  the  lineswitch  to  connect  his  line  to  an 


10- Selectors  per  Group 
10 -Connectors,  per  Group 


One  Level  of  Switch  Banks 
Symbol  Meaning 


Wiper 


.Bank 


Bank 


Bank 


Wiper 
Fig.    10. — Thousand-line  system. 

idle  selector.  He  then  pulls  "8"  on  his  calling  device,  in  response  to 
which  the  selector  lifts  its  wipers  to  the  eighth  level.  Then,  in  a  very 
brief  time,  the  selector  rotates  its  wipers  until  they  touch  the  contacts 
of  an  idle  trunk  line  (leading  to  an  idle  connector).  This  extends  the 
calling  line  to  a  connector  in  the  eighth  hundred  group.  The  subscriber 
then  dials  "0"  which  lifts  the  connector  wipers  to  the  "0"  or  tenth 
level.  He  now  dials  "5"  which  causes  the  connector  wipers  to  rotate 
to  the  fifth  set  of  contacts  on  the  "0"  level.  If  the  line  is  busy,  the 
connector  will  refuse  to  complete  the  connection,  and  will  send  back  a 
busy  tone  to  the  calling  telephone.  If  the  line  is  free,  the  connector  will 
complete  the  connection,  ring  the  bell  of  the  called  telephone  (No.  805) 
and  permit  conversation.  When  the  subscribers  hang  up  their  receivers, 
the  selector  and  connector  return  to  normal,  and  the  lineswitch  frees  the 
trunk  for  use  by  others. 


10 


A  I  jr()MA  TIC   TELKPIIO.X  Y 


The  wires  which  multiple  together  the  banks  of  adjacent  switches 
are  termed  collectively  "bank  wiring."  Thus  we  have  "lineswitch  bank 
wires,"  "selector  bank  wires,"  and  "connector  bank  wires." 

The  wires  which  multiple  together  different  groups  of  selector  banks 
are  called  "selector  bank  multiple." 

The  wires  which  lead  from  a  switch  to  a  different  kind  of  switch  ahead 
are  called  "  trunks."  Thus  we  have  "lineswitch  trunks,"  from  lineswitch 
banks  to  selector  jacks  (leading  to  wipers),  and  "selector  trunks"  from 
selector  banks  to  jacks  of  connectors. 

Wires  from  connector  banks  to  subscriber  lines  are  called  "normals." 


SYSTEMS  LARGER  THAN  ONE  THOUSAND  LINES 

It  is  useful  to  view  an  automatic  exchange  as  a  network  of  trunks 
which  connect  switching  junctions.  It  makes  easier  the  explanation  of 
the  larger  systems. 

In  its  simplest  form  we  may  consider  a  100-line  exchange  (Fig.  11) 
as  a  single  switching  junction  C  between  a  number  of  telephone  lines,  T. 
The  actual  junction  is  the  multipled  banks  of  the  connectors  of  Fig.  2. 
We  will  call  +,Ss  a  "junction"  because  it  is  a  point  where  the  various 
subscriber  lin      ire  connected  to  each  other. 


Fig.    11. — Hundred-line  exchange 
(one  junction). 


Fig.      12. — Hundred-line     exchange 
with  lineswitches  (two  junctions). 


A  100-line  exchange  with  lineswitches  (Fig.  12)  is  composed  of 
two  switching  junctions  with  suitable  connecting  links.  The  banks  of 
the  connectors  C  are  the  junctions  for  the  subscriber  lines  TT  and  the 
wires  leading  to  the  lineswitches  L.  (Compare  Fig.  8.)  The  link  which 
carries  thj  calls  to  the  lineswitches  is  composed  of  as  many  circuits  as 
there  are  subscriber  lines  TT.  The  link  which  ties  the  lineswitches  to  the 
connectors  has  in  it  as  many  circuits  and  connectors  as  are  necessary 
to  carry  the  traffic.  Each  part  of  this  link  must  be  thought  of  as  includ- 
ing the  lineswitch  trunk  and  its  connector  switch  clear  to  the  tips  of  its 
wipers. 

The  lineswitch  is  a  switching  junction  which  resembles  a  reducer  in 
a  pipe  line.  It  merely  reduces  a  large  number  of  circuits  to  a  smaller 
number  of  circuits. 


T RUN KING 


11 


The  1000-line  exchange  of  Fig.  13  has  three  kinds  of  switching 
junctions.  Each  group  of  subscriber  lines  TT  comes  in  to  its  own  con- 
nector junction  C,  so  that  C  is  able  to  distribute  calls  to  its  own 
subscribers.  There  is  another  switching  junction,  S,  which  is  able  to 
distribute  traffic  to  all  of  the  100-line  groups,  that  is,  to  all  of  the 
junctions  C.     Only  three  C  junctions  are  shown,  but  ten  must  be  imagined. 

The  junction  S  is  the  multipled  banks  of  all  the  selectors.  (Compare 
with  Fig.  10.)  The  links  which  connect  S  to  the  various  junctions  C 
are  selector  trunks,  including  each  connector  to  its  wipers. 

The  junction  S  receives  calls  from  all  the  junctions  L  (lineswitches). 
Each  link  between  one  L  and  S  is  composed  of  lineswitch  trunks  and 
selectors  to  their  wipers. 


Fig.   13. — Thousand-line  exchange 
(three  kinds  of  junctions). 


Fig.   14. — Thousand-line  exchange 
(re-arrangement) . 


One  group  of  lineswitches  and  their  trunks  may  serve  more  than  100 
subscriber  lines.  (Fig.  14.)  In  this  illustration,  one  lineswitch  junction 
L  handles  all  the  traffic  originated  by  200  lines.  The  rotary  lineswitch 
with  25  trunks  on  its  bank  can  handle  such  cases.  It  may  even  take  care 
of  300  or  more  lines. 

Ten  Thousand -line  System. — A  10,000-line  exchange  is  made  up  of 
groups  of  groups.  Ten  100-line  groups  or  switchboards  make  up  a  1000- 
line  groups.  Ten  of  these  1000-line  groups  make  up  the  entire  exchange. 
(See  Fig.  15.)  Only  three  100-line  boards  in  each  thousand  and  only  three 
thousands  are  shown,  but  the  reader  will  remember  that  the  full  size  is 
ten  of  each. 

Each  100-line  unit  has  its  own  connector  junction  C  as  in  smaller 
systems.  It  consists  of  as  many  connectors  as  are  necessary  to  dis- 
tribute the  traffic  going  to  the  100  subscribers  TT. 

Each  thousand  has  its  own  switching  junctions  2-S  which  distributes 
traffic  to  each  of  the  100-line  boards.  It  consists  of  the  bank  multiple 
of  "second  selectors."  These  second  selectors  perform  the  same  func- 
tions as  the  first  selectors  in  the  1000-line  system  (Fig.  10  and  Fig.  13) 


12 


AUTOMATIC  TELEPHONY 


in  that,  they  trunk  to  all  the  hundreds  of  a  given  thousand.  Usually 
there  arc  as  many  second  selectors  for  a  1000-line  group  as  there  are 
connectors  in  that  group.  Ten  100-line  boards,  each  having  ten  connec- 
tors, require  100  second  selectors,  whose  banks  form  the  "2-S"  for  the 
thousand. 

Each  junction  2-$  must  receive  traffic  from  all  the  thousands  in  the 
exchange.  Hence  on  the  one  side  the  links  (composed  of  first  selector 
trunks)  converge  from  all  the  thousands,  and  on  the  other  side  they 
diverge  to  the  separate  100-line  boards  within  the  one  thousand. 


Fig.   15. — Ten-thousand-line  exchange  (network  form). 

There  are  as  many  first-selector  junctions  (1-S)  as  there  are  junctions 
(2-*S).  Each  junction  (1-*S)  consists  of  the  banks  of  first  selectors  which 
are  multipled  together. 

It  is  necessary  that  the  traffic  coming  from  the  various  thousands 
shall  be  distributed  so  as  not  to  overload  any  one  junction  (1-S).  To 
secure  this,  each  one  of  them  receives  traffic  from  all  the  thousands. 
Thus,  the  junction  marked  A  (1-S)  receives  traffic  from  all  the  lineswitch 
units  marked  A(L).  The  junction  B(l-S)  receives  calls  from  all  the 
junctions  B(L).  This  arrangement  is  continued  throughout  all  the  ten 
thousands. 

Each  100-line  unit  C  is  shown  as  having  its  own  group  of  lines  witches 
L.  This  may  or  may  not  be  the  case  in  practice.  Several  hundreds 
may  use  the  same  group  of  lineswitches,  especially  if  they  are  of  the  ro- 
tary type.     A  given  hundred  may  be  served  by  two  or  more  groups  of 


TRUNKING  13 

lineswitches,  as  is  often  done  with  Keith  lineswitches,  if  the  traffic  is 
heavy.  Finally,  the  lineswitches  may  be  grouped  without  any  relation 
to  the  groups  of  subscriber  lines. 

The  trunking  described  above  is  shown  in  more  detail  in  Fig.  16. 
The  actual  apparatus  and  trunks  are  indicated  instead  of  the  junctions 
and  links  which  represent  them.  Three  out  of  the  ten  different  thou- 
sands are  shown,  also  three  out  of  the  ten  hundreds  in  each  thousand. 

All  of  the  lineswitches  are  in  a  row  at  the  left.  The  first  selectors 
and  connectors  in  groups  or  "sections"  of  ten  each  alternate  with  each 
other  in  the  second  column.  The  second  selectors  are  at  the  right,  ar- 
ranged in  sections  of  ten  switches  each. 

In  the  banks  of  each  section  of  ten  connectors  we  recognize  a  junction 
C  of  the  network.  These  connectors  deliver  calls  to  the  100  lines  to  which 
they  belong. 

In  the  banks  of  the  second  selectors  of  a  1000-line  group  we  can 
recognize  a  junction  2-S  of  the  network.  All  ten  sections  (100  second 
selectors)  are  multipled  together.  They  are  usually  lettered  A  to  J 
inclusive.  This  bank  multiple  binds  together  the  different  hundreds  in 
a  given  thousand. 

The  banks  of  the  first  selectors  are  also  multipled  together  so  as  to 
form  switching  junctions  1-S.  One  section  in  each  thousand  goes  into 
the  making  up  of  a  given  junction  1-S. 

The  ten  sections  of  first  selectors  in  a  given  thousand  are  lettered 
from  A  to  J  inclusive.  Since  there  are  ten  thousands  in  the  exchange, 
there  are  ten  A  sections,  ten  B  sections,  etc. 

In  order  to  distribute  the  traffic,  all  the  A  sections  of  first  selector 
banks  are  multipled  together  to  form  one  junction.  This  appeared  as 
A(1-*S)  in  Fig.  15.  All  the  B  first-selector  banks  likewise  are  multipled 
to  form  another  junction.  It  appeared  as  B(l-S)  in  Fig.  15.  In  this 
way  ten  bonds  are  formed  which  tie  together  all  the  ten  thousands. 

As  illustrated  in  Fig.  16,  each  first-selector  section  (ten  switches) 
receives  its  traffic  from  a  lineswitch  board  which  belongs  to  a  given  hun- 
dred. In  a  certain  sense  that  particular  100  lines,  its  lineswitches, 
first  selectors,  and  connectors,  may  be  said  to  belong  together.  But  this 
relationship  is  binding  only  for  the  connectors.  The  lineswitches  and 
the  first  selectors  may  be  grouped  quite  differently. 

The  first  selector  banks  are  trunked  to  the  second  selectors  according 
to  levels  and  thousands,  and  according  to  like  sections. 

The  first  level  of  all  first  selector  banks  must  trunk  to  second  selectors 
in  the  first  thousand,  second  level  to  second  selectors  in  the  second  thou- 
sand, third  level  to  the  third  thousand,  etc. 

The  trunks  from  A  first  selector  banks  are  trunked  to  A  second  selec- 
tors in  all  the  thousands.  B  first  selector  banks  trunk  to  B  second  selec- 
tors in  all  the  thousands.     This  grouping  applies  to  all  the  sections. 


14 


AUTOMATIC  TELEPHONY 


First  Selectors 


100  Trun 


L.Sw. 


I200     £ 


2IOO    q 


Connector 
Fio.  16. — Ten-thousand-line  exchange  with  lineswitches. 


T  RUN  KING  15 

The  combination  of  the  above  two  schemes  results  in  the  arrangement 
of  trunks  shown.  Consider  the  A  first  selectors.  From  the  first  level 
of  their  banks  run  ten  trunks  to  the  A  section  of  second  selectors  in  the 
first  thousand.  From  the  second  level  of  A  first  selector  banks  run  ten 
trunks  to  the  A  second  selectors  in  the  second  thousand.  Likewise  the 
third  level  trunks  to  A  second  selectors  in  the  third  thousand.  This 
is  continued  throughout  all  the  levels. 

Consider  the  B  first  selectors.  Their  first  bank  level  trunks  to  the 
B  section  of  second  selectors  in  the  first  thousand.  The  second  level 
trunks  to  B  second  selectors  in  the  second  thousand.  The  third  level 
leads  to  B  second  selectors  in  the  third  thousand.  This  is  continued 
to  all  levels  and  thousands. 

In  this  way  all  first  selector  sections  of  a  given  letter  trunk  to  all 
thousands  to  second  selector  sections  bearing  the  same  letter.  Also  each 
second  selector  section  of  a  given  letter  receives  traffic  from  all  thousands 
from  first  selector  sections  bearing  the  same  letter.  The  traffic  is  effec- 
tively mingled. 

We  will  trace  three  calls,  illustrating  three  different  conditions  in 
trunking  which  will  be  met  in  actual  traffic.  They  are  a  call  from  a 
number  to  another  number  in  the  same  hundred,  from  a  number  to 
another  in  a  different  hundred  in  the  same  thousand,  and  between  num- 
bers in  different  thousands. 

Let  No.  2125  call  No.  2170.  On  taking  the  receiver  from  the  hook,  the 
line  switch  No.  25  on  the  "2100"  board  connects  the  line  to  an  idle  first 
selector  on  section  A.  Pulling  "2"  on  the  dial  lifts  the  wipers  of  that  first 
selector  to  the  second  level  and  automatically  picks  out  an  idle  trunk 
leading  to  section  A  in  the  same  thousand.  The  second  operation  of  the 
calling  device  sends  one  impulse,  which  lifts  the  wipers  of  this  second 
selector  to  the  first  level  and  causes  them  to  seize  an  idle  trunk  to  a  con- 
nector in  the  hundred,  known  as  the  "2100"  hundred.  The  last  two 
pulls  of  the  dial,  7  and  0,  lift  the  connector  wipers  to  the  seventh  level  and 
rotate  them  to  the  tenth  set  of  contacts  in  that  level,  which  is  numbered 
"70."  The  line  now  extends  through  the  normal  cable  to  the  jacks  of 
lineswitch  70  and  out  to  the  called  telephone. 

Let  telephone  No.  2130  call  No.  2241.  The  call  will  extend  from  line- 
switch  30  on  the  2100  board  to  an  idle  first  selector  on  the  "A"  section. 
Thence  it  is  trunked  through  the  second  bank  level  to  a  second  selector 
on  section  A  in  the  same  thousand.  From  here  it  goes  through  the 
second  bank  level  to  a  connector  attached  to  the  "2200"  board,  and  from 
the  forty-first  set  of  contacts,  through  the  normal  cable  to  lineswitch 
jacks,  41  and  out  to  the  telephone    No.  2241. 

If  telephone  No.  3255  calls  No.  1267,  the  call  will  be  routed  as  follows: 
lineswitch  55  on  the  "3200"  board,  idle  first  selector  on  "B"  section, 
first  level  of  first  selector  banks  to  idle  second  selector  on  " B"  section 


16  AUTOMATIC   TELEPHONY 

of  first  thousand,  second  level  of  this  second  selector  to  idle  connector  on 
"  1200"  board,  and  by  the  usual  manner  to  the  line  No.  1267. 

The  Secondary  Lineswitch. — The  object  of  the  secondary  lineswitch 
is  to  reduce  the  number  of  trunks  and  trunking  apparatus  by  taking  ad- 
vantage of  the  economies  offered  by  larger  groups  of  trunks.  It  has 
been  proved  by  experience  that  small  trunk  groups  can  not  handle  as 
many  calls  per  trunk  as  large  groups  can  handle.  The  saving  effected  by 
large  trunk  groups  is  treated  more  at  length  in  the  chapter  on  "Traffic," 
to  which  the  reader  is  referred. 

The  secondary  lineswitch  is  introduced  in  two  places,  (1,  local)  be- 
tween subscriber  lineswitches  and  first  selectors  to  reduce  the  number  of 
selectors  required,  and  (2,  outgoing)  between  selector  banks  and  out- 
going trunks  to  other  offices  of  a  multi-office  exchange  to  reduce  the 
number  of  inter-office  trunks,  the  incoming  selectors  in  the  distant  office, 
and  the  impulse  repeaters  associated  with  each  trunk  in  the  sending 
office. 

The  general  principle  of  trunking  by  primary  and  secondary  line- 
switches  is  shown  by  Fig.  17.  The  subscribers'  lines  are  attached  to  the 
primary  lineswitches.  The  secondary  lineswitch  boards  are  shown  at 
the  right. 

As  many  as  2500  lines  have  been  put  in  one  large  group  or  division, 
though  on  account  of  the  space  only  1000  have  been  shown  in  the  figure. 
If  the  traffic  from  each  100  lines  is  not  too  great,  only  ten  trunks  will  be 
allowed  per  100,  as  illustrated. 

The  trunks  from  the  banks  of  the  primary  lineswitches  run  to  the 
secondary  lineswitches,  indicated  by  the  arrows  marked  "plungers." 
From  the  banks  of  the  secondary  lineswitches  the  trunks  run  to  first 
selectors,  ten  for  each  section  of  secondaries.  They  are  lettered  from  A 
to  J  as  usual,  and  their  multipling  is  done  in  accordance  with  the  prac- 
tices already  described. 

It  should  be  observed  that  not  more  than  one  trunk  from  a  primary 
board  runs  to  a  given  secondary  section.  Examining  primary  board  1, 
notice  that  trunk  1  (beginning  at  the  top)  runs  to  secondary  section  1, 
trunk  2  to  secondary  section  2,  etc.  This  is  one  of  the  prime  facts  of  the 
arrangement,  and  upon  it  depends  the  effectiveness  of  the  large  trunk 
group.  By  giving  each  primary  board  a  trunk  into  each  of  the  ten 
secondary  sections,  we  render  all  the  first  selectors,  100  of  them,  avail- 
able to  all  the  subscribers;  also,  if  all  the  first  selectors  of  a  given  secondary 
shelf  become  bus}'  it  renders  useless  not  more  than  one  trunk  from  each 
primary  board  or  section. 

This  arrangement  may  be  seen  in  both  kinds  of  lineswitches.  If  we 
consider  any  primary  lineswitch,  we  shall  see  that  it  has  a  choice  of  ten 
trunks,  any  one  of  which  will  serve  equally  well.  It  has  to  share  this 
choice  with  from  49  to  99  other  lines.     When  the  subscriber's  line, 


T RUN KING 


17 


Primary  Line  SwS. 
Phingers    ,-BanM 


Secondary  line  Sws 
Plungers 


Fig.  17. — Primary  and  secondary  trunking  (distributive). 


18 


A  U  TOM  A  TIC  TELEPHON  Y 


through  its  primary  lineswitch,  has  seized  one  of  the  ten  trunks,  it  is 
thus  extended  to  a  lineswitch  on  one  of  the  secondary  boards.  Herd 
again  it  has  a  choice  of  ten  trunks  any  one  of  which  will  serve  equally 
well.  It  has  to  share  this  choice  with  a  number  of  other  secondary  line- 
switches.  But  by  the  double  choice  of  ten  trunks  each  we  secure  the 
ability  to  seize  any  one  of  100  trunks,  each  leading  to  first  a  selector. 
If  the  traffic  from  100  subscribers'  lines  is  too  great  to  be  carried  by 
ten  trunks,  it  is  customary  to  divide  a  primary  lineswitch  board  into 
two  parts  of  50  lines  each,  and  to  give  each  part  from  seven  to  ten  trunks. 


MULTI-OFFICE  EXCHANGE 

The  automatic  system  is  easily  divided  into  a  number  of  offices  with 
out  increasing  the  number  of  digits  in  the  call  number.  This  may  be 
illustrated  by  dividing  a  1000-line  exchange  into  two  parts.  We  can 
put  the  first,  second,  third,  fourth  and  fifth  hundreds  in  one  office,  and 
the  rest  of  them  in  another  office.  For  simplicity  assume  only  four 
hundreds,  placing  the  first  and  second  hundreds  in  the  first  office  and  the 
fourth  and  fifth  hundreds  in  the  second  office.     (See  Fig.  18.) 


Fi'rst  Office 


Second  Office 


First  Hundred 
Subs.  Lines 


Second  Hundred 
Subs.  Lines 


Fourth  Hundred 
Subs.  Lines 


Fifth  Hundred 
Subs- Lines 


LineSw. 


LineSw 


LineSw 


LineSw. 


Fig.    18. — Thousand-line,  2-office  exchange. 

We  will  assume  that  the  traffic  division  is  ideal,  i.e.,  that  there  is  no 
community  factor  and  therefore  half  of  the  originating  traffic  in  each  office 
goes  to  lines  in  the  same  office  and  the  other  half  to  the  other  office.  This 
is  necessary  in  opening  up  the  subject. 

In  each  office  the  first-selector  banks  are  multipled  to  each  other, 
but  there  is  no  common  multiple  between  the  two  offices.  In  the  first 
office,  levels  1  and  2  run  to  local  connectors  in  the  corresponding  hun- 
dreds. Trunks  from  the  fourth  and  fifth  levels  run  to  incoming 
connectors  in  the  second  office,  terminating  in  the  fourth  and  fifth 
hundreds.     In  the  second  office,  the  reverse  is  the  case.     The  first  and 


T  RUN  KING 


19 


second  levels  run  to  the  distant  office  to  incoming  connectors,  while  the 
fourthand  fifth  levels  carry  the  local  traffic  to  local  connectors  in  the 
fourth  and  fifth  hundreds.     This  is  one  way  of  handling  such  a  case. 

The  10,000-line  system  may  also  be  divided  into  two  offices  of  5000 
lines  each,  placing  the  first,  second,  third,  fourth  and  fifth  thousands  in 


Is+.Sel. 


Znd.Sel. 


Connec+or 

Is+Hund.   \ 

?nd.  Hund. 

__;  Zrd.  Hund. 

BS  4  th.  Hund. 

ls+.Hund.\ 
?nd.  Hund.  I 


Fig.    19. — Ten-thousand-line,  2-office  exchange. 

one  office,  and  the  rest  in  the  other  office.  It  is  however,  not  essential 
that  consecutive  thousands  be  placed  in  one  office.  We  could  just  as 
well  put  the;  second,  fifth,  ninth  and  any  other  thousands  in  one  office, 
and  the  rest  in  the  other. 

The  10,000-line  system  has  no  one  first  selector  bank  multiple.  As 
was  described  before,  all  the  "A"  sections  of  first  selectors  are  multipled 
together,  all  the  "B"  sections  are  multipled  in  another  group,  etc.     For 


20  AUTOMATIC  TELEPHONY 

this  reason,  the  division  into  two  offices  can  not  be  handled  exactly  as 
was  done  in  the  case  of  the  1000-line  exchange. 

We  will  illustrate  by  taking  4000  lines  and  dividing  them  into  two 
offices.  (Fig.  19.)  The  first  office  has  the  first  and  second  thousands, 
and  the  second  office  the  third  and  fourth  thousands. 

Each  switch  symbol  represents  a  section  of  ten  switches.  The 
first  thousand  has  ten  hundreds,  of  which  only  four  are  represented,  and 
each  hundred  has  ten  connectors.  The  second  selector  banks  are  multi- 
pled  in  common  within  the  thousand  to  which  they  belong.  The  various 
levels  are  trunked  to  connectors  as  usual. 

We  will  assume  an  ideal  traffic,  i.e.,  that  the  division  of  originated 
traffic  is  proportional  to  the  number  of  lines  in  the  two  offices.  In  this 
case,  half  the  traffic  originating  in  the  first  office  will  be  trunked  out  to 
the  other  office,  and  half  will  remain  as  local  traffic.  Accordingly,  as 
one  means  of  grouping,  Ave  may  multiple  together  the  banks  of  all  the  A 
and  B  sections  of  first  selectors  so  that  they  will  trunk  together.  In  the 
same  way,  we  will  place  the  C  and  D  sections  together,  etc. 

In  the  first  office,  levels  one  and  two  are  local,  and  go  to  local  second 
selectors  in  the  thousands  to  which  they  belong.  The  third  and  fourth 
levels  lead  to  outgoing  trunks  to  the  second  office  where  they  terminate 
in  incoming  second  selectors,  whose  banks  are  multipled  to  the  local 
second  selectors  in  the  appropriate  thousands.  In  every  case,  the  local 
and  incoming  second-selector  banks  are  multipled  together. 

The  traffic  may  go  to  the  first  selectors  in  any  way  whatever.  It  is 
not  material  to  the  scheme  of  inter-office  trunking  whether  the  traffic 
goes  directly,  through  lineswitches,  or  through  primary  and  secondary 
lineswitches.  The  multipling  of  the  banks  of  first  selectors  would  in 
every  case  be  carried  out  in  such  a  manner  as  to  secure  as  thorough  a 
mixing  up  of  the  traffic  as  possible. 

In  a  fully  equipped  10,000-line  system,  divided  ideally  into  two  offices 
of  5000  lines  each,  there  would  be  500  first  selectors  on  the  10  per  cent 
basis.  The  first  selector  bank  group  (five  A  sections  and  five  B  sections) 
would  comprise  100  selectors,  trunking  into  ten  trunks  from  a  given 
level,  or  50  trunks  from  the  five  levels  which  go  to  the  other  office.  Each 
of  these  ten-trunk  groups  go  to  a  particular  thousand  in  the  other  office. 
There  will  be  five  such  first  selector  bank  groups  of  50  trunks  each, 
making  a  total  of  250  trunks  one  way  from  one  office  to  the  other. 

The  outgoing  traffic  is  apportioned  among  the  thousands  before  it 
leaves  the  originating  office;  it  is  the  work  of  the  first  selectors  We 
may  regard  the  250  trunks  referred  to  above  as  really  being  in  five  groups, 
one  going  to  each  of  the  thousands  in  the  distant  office. 

Outgoing  Trunk  Secondary  Lineswitches. — The  efficiency  of  inter- 
office trunks  is  increased  if  the  ten-trunk  groups  can  be  merged.  The 
secondary  lineswitch  is  used  for  this  purpose. 


T RUN KING 


21 


The  two-office  system  of  Fig.  19  would  require  50  trunks  from  the 
first  office  to  each  of  the  thousands  in  the  second  office.  Each  group  of 
50  is  divided  into  five  groups  of  ten  trunks  each.  Thus  all  of  the  A  and 
B  first  selectors  in  the  first  office,  trunk  through  ten  trunks  to  the  C 
section  of  second  selectors  in  the  third  thousand  (second  office).  The  C 
and  D  first  selectors  in  the  first  office  have  a  different  group  of  ten  trunks 
to  the  same  third  thousand  (section  D  of  second  selectors).  If  the  two 
offices  have  five  complete  thousands  each,  it  takes  five  such  groups  of 
ten  trunks  -to  carry  the  traffic  from  the  first  office  to  the  third  thousand 
alone. 


"A"0ffi'ce  -First  and  Second  Thousands 
lOTrks. 


Secondare      „  „ 

Line  Switches      B  Office -Third  and  Fourth  Thousands 

Switches,..     .Sanks 


lOTrks 


lOTrks 


IQTrks 


lOTrks 


Local  2nd. 
Selectors 


-*~  Upcoming  Znd~& 
§. =  [  selectors     .t 


-+ —  wot  Required 


' I  Local c 'net 

-  ^Selectors 


' —  \lncoming?nd.  i 
.=  [Selectors       ■ 


Not  Required 


Fig.  20. — Outgoing  secondary  lineswitches,  10,000-line,  2-office  exchange. 

The  outgoing  trunk  secondary  lineswitch  renders  all  trunks  available 
to  all  selectors  in  a  group.  (Fig.  20.)  At  the  left  are  representations 
of  the  first  selectors  in  the  first  office  and  their  outgoing  secondary  line- 
switches.  The  incoming  second  selectors  of  the  second  office  are  at  the 
right.  We  are  assuming  that  30  trunks  will  handle  the  traffic  which 
would  require  50  trunks  without  the  secondaries. 

For  each  thousand  in  the  second  office  we  provide  three  sections  of 
secondary  lineswitches  in  the  first  office.  To  them  we  distribute  the 
trunks  from  first  selectors  by  levels. 

Each  ten-trunk  group  is  divided  as  evenly  as  possible  among  the  three 
sections  of  secondaries.  This  means  three  lineswitches  in  one  section, 
three  in  another  section,  and  four  in  another  section. 


22  AUUOMATIC  TELEPHONY 

This  is  not  the  only  way  to  connect  selectors  to  outgoing  secondaries, 
but  in  general  it  illustrates  the  method. 

One  Hundred  Thousand-line  System. — It  is  usual  in  a  system  of  this 
size  to  assign  one  digit  of  the  call  number  to  an  office.  This  results  in 
the  ideal  arrangement  of  ten  offices,  each  having  a  capacity  of  10,000 
lines.     Variations  will  be  discussed  later. 

The  essentials  of  the  inter-office  trunking  are  shown  in  Fig.  21.  The 
horizontal  broken  line  divides  office  No.  1  from  office  No.  2.  These 
two  offices  are  assumed  to  be  part  of  a  100,000-line  system  with  an  ulti- 
mate capacity  of  10,000  lines  in  each  office,  and  there  are,  therefore, 
line-switches,  first,  second  and  third  selectors  and  connectors.  The 
subscribers'  lines  pass  through  lineswitches  directly  to  first  selectors 
arranged  in  sections  A,  B,  etc.  Since  inter-office  trunking  begins  at  the 
first-selector  banks,  we  need  not  bother  ourselves  as  to  whether  there  are 
primary  and  secondary  line-switches. 

It  is  assumed  that  the  student  is  familiar  with  the  ordinary  inter- 
office network  in  which  there  is  a  group  of  direct  trunks  from  each  office  to 
every  other  office  in  the  exchange.  This  gives  a  total  of  N(N-l)  groups. 
Automatic  switching  in  the  ideal  exchange  which  we  are  describing 
links  the  offices  together  by  such  a  network. 

Since  the  first  selectors  are  the  office-choosing  switches,  the  first- 
selector  bank  levels,  excepting  the  level  corresponding  to  the  home  office, 
will  be  trunked  to  other  offices.  In  office  No.  1,  level  1  is  trunked  to 
local  second  selectors  which  distribute  the  traffic  among  the  different 
thousands.  Level  2  of  the  first  selector  banks  is  trunked  through  cables 
to  office  No.  2,  where  the  trunks  end  in  incoming  second  selectors.  If 
there  be  other  offices  in  the  system,  levels  3,  4,  5,  etc.,  will  lead  in  a  similar 
manner  to  their  respective  offices.  In  office  No.  2,  level  1  from  the  first 
selectors  is  trunked  to  incoming  second  selectors  in  office  No.  1.  The 
trunks  from  the  second  level  of  first  selectors  in  office  No.  2  are  carried  to 
local  second  selectors  in  office  No.  2. 

The  "first  selector"  in  this  system  did  not  exist  in  the  smaller  systems 
just  described.  The  name  was  there  but  the  function  was  not.  The 
first  selector  in  a  10,000-line  system  selects  "thousands."  This  same 
function  in  the  100,000-line  system  is  performed  by  the  second  selector. 
It  is  the  position  of  any  switch  in  the  chain  of  selections  that  gives  to  it 
the  name.  The  first  to  be  used  is  called  "first  selector,"  the  second  to 
be  used  is  a  "second  selector,"  and  so  on. 

The  first  selector  banks  are  multipled  together  so  as  to  mingle  the 
traffic  and  prevent  any  one  group  of  ten  trunks  from  being  over  loaded. 
All  A  sections  may  be  multipled  together,  all  B  sections  together,  etc.; 
or  the  A  and  B  sections  may  be  tied  together,  the  C  and  D,  etc.  Some- 
times the  multipling  is  A  to  F,  B  to  G,  etc.  It  is  not  necessary  for  the 
outgoing  levels  to  be  multipled  in  the  same  way  as  the  local  banks,  nor 


T RUN KING 


23 


first  Selectors 


Second  Selectors  Third  Selectors  Connect fors 

(Local) 


=g^ 


Fig.   21. — Inter-office  trunking  (100,000-line  system). 


24  AUTOMATIC  TELEPHONY 

all  the  local  banks  in  the  same  way.  It  is  entirely  a  matter  of  traffic 
requirements. 

Economies  can  be  secured  by  bringing  into  one  group  all  the  trunks 
from  one  office  to  another.  To  secure  this  end  outgoing  secondary 
lineswitches  are  interposed  between  the  first  selector  banks  and  the 
outgoing  trunks  in  the  originating  office.  The  trunks  from  any  given 
section  of  first  selectors  are  distributed  among  several  sections  of  secon- 
dary line  switches  in  order  that  each  section  of  first  selectors  may  have 
access  to  all  the  trunks  leaving  the  office.  In  this  way  the  several  groups 
of  trunks  leaving  the  secondary  lineswitches  become  in  fact  a  common 
group.  In  case  any  one  section  becomes  busy,  the  traffic  will  be  carried 
by  the  remaining  sections  and  each  section  of  first  selectors  will  have  only 
one  trunk  made  busy. 

Repeaters  are  inserted  in  the  trunks  just  before  they  leave  the  origi- 
nating office.  They  reduce  the  number  of  trunk  wires  to  two  instead  of 
three,  besides  affording  other  advantages  which  are  discussed  in  the  text 
which  treats  of  the  circuits. 

The  second  selectors  in  each  office  perform  the  same  function  which  is 
performed  by  the  first  selectors  in  a  10,000-line  system,  that  is  to  say,  they 
distribute  the  traffic  to  the  different  thousands. 

The  multiple  cables  connecting  the  banks  of  the  second  selectors  form 
the  connecting  link  between  the  different  thousands.  By  multipling 
together  sections  having  the  same  letter,  the  traffic  from  outside  offices 
coming  through  incoming  second  selectors  is  mingled  with  the  local 
traffic. 

Large  Trunk  Groups. — In  the  network  of  a  large  multi-office  exchange 
it  often  occurs  that  the  traffic  between  two  offices  requires  a  large  number 
of  trunks.  Theoretically  at  least  the  number  of  calls  which  each  trunk 
will  carry  increases  with  the  size  of  the  group  to  a  very  large  number. 
As  many  as  1000  trunks  in  one  group  have  been  considered.  It  is 
possible  to  put  together  apparatus  to  secure  this  large  a  group.  But 
it  is  doubtful  if  it  is  advisable.  Indeed,  there  are  many  who  think 
that  a  practical  limit  is  reached  at  250  trunks. 

The  usual  selector  switch,  having  ten  trunks  in  each  level  of  its  bank 
may  have  access  to  100  trunks  by  connecting  each  selector-bank  point  to 
a  Keith  secondary  lineswitch.  This  was  illustrated  in  Fig.  20.  In  this 
case  the  selectors  of  one  group  would  have  but  one  secondary  lineswitch 
in  each  section  of  secondaries,  and  there  would  be  ten  sections  of  secon- 
daries, each  feeding  traffic  into  ten  trunks. 

By  using  a  selector  which  has  20  trunks  on  each  level  of  its  bank,  the 
outgoing  trunk  group  is  increased  to  200.  There  is  no  loss  of  speed 
in  trunk  selection  by  this  method. 

The  ton-trunks-per-level  selector  used  in  connection  with  the  rotary 
lineswitch  (25  trunks  on  the  bank)  gives  a  maximum  of  250  trunks  in  one 


T RUN KING 


25 


group.  The  distribution  of  trunks  from  the  selector  banks  to  the  line- 
switches  is  made  as  described  for  the  Keith  switches. 

The  selector  whose  bank  has  20  trunks  per  level  together  with  the 
rotary  lineswitch  gives  a  group  of  500  trunks. 

The  20-trunk  selector  with  a  rotary  lineswitch  having  50  trunks  on  its 
bank  give  a  maximum  of  1000  trunks  in  a  group. 

All  the  combinations  mentioned  involve  no  new  principles  of  struc- 
ture, mechanically  or  electrically. 

Exchanges  Having  More  Than  Nine  or  Ten  Offices. — In  an  exchange 
which  has  more  than  nine  or  ten  central  offices,  we  can  not  take  the  first 
selectors  for  office  selectors  alone.  This  is  because  we  can  not  set  aside 
one  first-selector  level  for  one  office,  for  there  are  more  offices  than  levels. 
It  is  necessary  to  increase  the  availability  in  one  of  two  ways. 


Out  to 
Other  Offices 


3rd. Selectors 


3rd.  Thou. 


Office-  45000-4800Q | 

Fig.   22. — One  first-selector  level  for  two  offices 


r 

I    Office  -42  OOP-  43000 


We  may  let  two  offices  share  one  level  of  first  selectors.  (Fig.  22.) 
Assume  a  100,000-line  system.  There  are  five  digits  in  the  call  number. 
The  illustration  concerns  the  fourth  level. 

Numbers  in  the  45,  46,  47  and  48  thousands  are  given  to  one  office 
(at  the  left).  Numbers  in  the  42  and  43  thousands  are  placed  in  another 
office,  usually  near  by.  From  the  first  selector  banks  in  the  45,000- 
48,000  office  the  trunks  run  to  other  offices,  except  that  the  fourth  level  is 
local.     These  fourth-level  trunks  run  to  local  second  selectors. 

Levels  five  to  eight  on  the  second  selectors  run  to  third  selectors 
which  are  grouped  according  to  thousands.  Their  banks  in  turn  lead 
to  connectors  by  hundreds.  Incoming  trunks  from  other  offices  in  the 
exchange  terminate  on  incoming  second  selectors  whose  banks  are 
multipled  with  the  local  second  selectors. 

Levels  two  and  three  are  trunked  out  to  the  associated  office.  There 
they  terminate  in  third  selectors  for  the  second  and  third  thousands. 

The  second  selectors  at  whose  banks  occur  the  division  between 


26 


AUTOMATIC!  TELEPHONY 


offices  may  be  located  anywhere,  the  question  being  one  of  local  condi- 
tions. They  may  be  in  one  of  the  two  offices,  as  illustrated.  They  may 
be  in  a  third  office.  (Fig.  23.)  In  this  case  the  traffic  from  the  four 
offices  at  the  left  divides  at  office  A  for  the  42-43  thousand  office  and  the 
45^48  thousand  office.  When  any  subscriber  dials  "4,"  his  first  selector 
trunks  the  call  to  a  second  selector  at  A.  If  now  he  pulls  "2"  or  "3" 
that  selector  will  trunk  the  call  to  the  lower  office  at  the  right.     But  if 


Fig.  23.— Traffic  divided  at 
intermediate  office. 


Fig.  24. — Traffic  divided  at  two  intermediate  offices. 


he  pulls  any  digit  from  "5"  to  "8"  inclusive,  that  selector  will  trunk 
the  call  to  the  upper  office. 

Again,  the  traffic  may  be  divided  at  two  intermediate  offices.  (Fig. 
24.)  In  this  case  the  traffic  from  offices  at  the  left  is  most  conveniently 
gathered  together  and  separated  at  office  A  while  that  coming  from  the 
offices  at  the  right  can  be  handled  better  at  office  B.  The  exact  routing 
of  inter-office  cables  is  not  indicated. 


First  Selectors 


Office  No.33 
Office  No.32 
Off  fee  No. 31 
Office  No.  23 

Off  ice  No.  22 
Office  No.  2/ 
Office  No.  13 

Office  No.  12 
Office  No.  II 


Fig.  25. — Direct  trunks  with  first  and  second  selectors. 


Another  way  of  handling  the  large  exchange  is  to  provide  first  selectors 
and  second  selectors  in  each  office,  with  direct  trunks  between  offices  in 
most  cases.  (Fig.  25.)  This  permits  the  manual  trunks  to  be  used  for 
automatic  without  rearrangement.  Direct  trunking  is  not  always  the 
most  economical,  but  in  changing  from  manual  to  automatic  the  existing 
trunk  links  must  often  be  used  until  the  more  economical  arrangement 
possible  with  automatic  can  be  secured. 


T RUN KING 


27 


A  better  way  is  to  divide  the  exchange  into  districts,  each  with 
a  number  of  offices.  (Fig.  26.)  Theoretically  there  may  be  ten  districts 
each  having  ten  offices,  or  100  offices  in  all.  In  the  illustration,  three 
districts  are  shown,  each  having  three  offices.  For  convenience,  calls  are 
assumed  to  originate  in  office  No.  11,  and  one  first  selector  shows  how  any 
number  of  first  selectors  will  act.  The  first  selectors  choose  the  districts. 
If  "  1 "  is  the  first  digit  of  the  call  number,  the  first  selector  will  take  the 
first  digit  of  the  call  number,  the  first  selector  will  take  the  first  level, 
which  leads  to  a  second  selector  whose  bank  trunks  to  third  selectors  in 
other  offices  in  the  same  district  (offices  12  and  13,  etc.)     If  the  first  digit 


District  Ho.1 


District  No.  5 
3rd.  Sels. 
==     Office  No. S3 


Fig.   26. — Large  exchange  divided  into  districts. 


is  "2,"  the  first  selector  will  secure  a  trunk  to  a  second  selector  in  another 
group,  whose  bank  is  trunked  to  third  selectors  in  district  2.  The  second 
pull  of  the  dial  lifts  the  wipers  of  this  second  selector  to  the  level  corre- 
sponding to  the  office  in  the  district. 

The  fundamental  principle  underlying  the  selection  of  offices  in 
districts  is  the  same  as  the  selection  of  a  100-line  group  out  of  1000  lines. 

Having  the  exchange  divided  into  districts,  it  is  more  economical  to 
locate  the  second  selectors  of  a  district  in  the  district  which  they  serve. 
(Fig.  27.)  In  this  case  the  trunks  from  first  selectors  to  second  selectors 
are  long  and  those  from  second  selectors  to  third  selectors  are  short.  The 
former  are  common,  the  latter  are  individual  to  the  office. 

If  a  subscriber  of  office  11  calls  13,  he  will  lift  the  wipers  of  his  first 
selector  to  the  first  level,  secure  a  trunk  to  a  second  selector  in  the  same 
district,  so  that  on  pulling  ''3,"  he  will  get  a  trunk  to  office  13.     These 


28 


AUTOMATIC  TELEPHONY 


second  selectors  may  be  located  in  office  11,  or  in  any  convenient  place  in 
district  1. 

If  the  subscriber  calls  "22"  the  first  selector  will  go  to  its  second  level, 
and  the  second  selector  in  district  2  will  be  lifted  to  its  second  level,, 
delivering  the  connection  to  office  22.  These  second  selectors  may  be 
located  anywhere  in  district  2,  the  place  depending  on  economy  of  trunks. 

It  is  usual  to  locate  these  second  selectors  in  the  most  convenient  of 
the  offices  of  the  district.  But  if  the  conditions  warrant,  they  may  be 
put  in  a  building  by  themselves,  with  no  local  subscriber  lines  at  all.  In 
this  case  it  is  a  "switching  office,"  pure  and  simple.  When  the  second 
selectors  are  located  in  an  office  which  serves  subscriber  lines,  the  office 
has  a  double  function,  one  of  which  is  switching  office. 


District  No. 5 
3rdSels. 


Office  No.  53 


Fig.  27. — Large  exchange,  second  selectors  in  their  own  districts. 

Tandem  trunking  is  the  term  applied  to  the  idea  of  selectors  located 
at  an  intermediate  office  to  switch  calls  between  other  offices.  The  term 
holds  in  both  the  cases  described  above. 

The  outgoing  trunks  from  an  office  need  not  be  arranged  the  same  as 
the  incoming  trunks.  There  is  much  freedom  as  to  their  arrangement. 
One  may  be  direct  trunks,  the  other  may  be  tandem  trunks. 

The  outgoing  traffic  from  a  district  may  often  be  combined  to  advan- 
tage by  grouping  the  second  selectors  at  one  place  in  the  district.  (Fig. 
28.)  Here  they  are  shown  as  located  in  office  11,  it  being  assumed  that 
this  office  is  where  it  can  best  gather  the  traffic  from  all  the  other  offices 
of  the  district.  This  renders  each  trunk  available  to  all  the  offices  of  the 
district.     It  is  a  form  of  tandem  trunking. 

The  switching-office  idea  and  tandem  trunking  may  be  carried  out 
regardless  of  districts.     (Fig.  29.)     As  long  as  we  are  gathering  together 


T RUN KING 


29 


originating  traffic,  we  may  combine  at  any  suitable  place  the  trunks  which 
come  from  any  offices  by  using  second  selectors  to  function  as  a  switching 
office.     It  gives  great  flexibility  to  the  system. 

Offices  and  districts  may  be  rearranged,  if  the  development  of  a  city 
does  not  follow  the  lines  which  were  apparent  when  the  exchange  was 
installed.     Suppose  (Fig.  30)  that  when  the  exchange  was  laid  out  it  was 


Di  st-ricfNoJ 


Fig.  28. — Combining  traffic  from  one  group  of  offices  to  another  group. 


thought  that  districts  2  and  3  would  ultimately  need  ten  10,000-line 
offices  each.  Accordingly,  offices  21,  22,  23,  etc.  to  29  and  20  were  set 
aside  for  district  2  and  offices  31,  32,  33,  etc.  to  39  and  30  were  set  aside 
for  district  3.  The  upper  part  of  the  illustration  shows  trunks  from  the 
west  offices  coming  to  second  selectors  in  the  switching  office  of  district  3 
and  similarly  to  district  2.  The  switching  office  of  district  2  has  also 
second  selectors  for  traffic  from  the  offices  which  lie  to  the  south  of  dis- 
trict 2. 

Anu  Office  i 1 

I  JL£       j  =  0fficeNo.?3  j 

|   — •->        ^L  Any  Office  ^^~_ZZ.~ZUZSZ-~ZJZ-y 

I l\  |  2~S  I   ^^         i      ^     „    ,J 

=  Office  No.  22  \ 

TT* —  J 

AjiiiPfffce 

-s   "  ^  ±g/  j     \  p-u -, 

I     Local  =/  1  \.      I       =  Office  No.2/\ 

— ->       i  u=^ i  \L>~" 

i i  i 1 

Fig.  29. — Combining  traffic  at  an  intermediate  office. 


But  suppose  that  district  2  fails  to  develop,  and  that  district  3  grows 
telephonically  beyond  the  ten  offices  which  were  assigned  to  it.  This 
can  be  corrected  without  changing  the  numbers  of  any  telephones. 

Move  part  of  the  switching  office  of  district  2  into  district  3  either 
in  its  switching  office  or  elsewhere.  The  former  is  shown.  The  second 
selectors  which  are  moved  are  marked  "i"  in  the  two  sketches. 


30 


A  UTOMA  TIC  TELEPHON  Y 


The  selectors  <ex"  lower  part  of  Fig.  30,  which  were  moved  from  dis- 
trict 2  to  district  3,  have  ten  levels,  part  of  which  are  occupied  by  trunks 
for  the  existing  offices  of  district  2.  Levels  1,  2,  and  3  are  shown.  They 
are  now  trunked  separately  to  the  offices  which  they  serve.  The  vacant 
levels  (4  and  5  are  shown)  are  now  trunked  to  new  offices  (24  and  25) 
which  are  opened  to  help  take  care  of  the  growth  in  district  3.  Thus 
district  3  has  annexed  part  of  the  offices  which  were  set  aside  for  district  2. 

Traffic  for  district  2  which  comes  from  south  offices  will  be  handled 
by  second  selectors  in  the  switching  office  as  before  the  change.     The 


Switching 
Office   2-$ 


From  West  Offices 


District  No. 5 


Switching  Office 
2-S  ?-S 


As  Laid  Out  and  Installed 


Distrjct  No.2 

r~  _ 

=  Office  21 


District  No.  3 


from  West 
;  Offices 


As  Changed  to  Meet  Conditions 


Switching  Office 
From 
South 
Offices 

Fig.   30. — Trunking,  inter-office,  switching  office  moved  to  re-arrange  districts. 


\\So 


three  levels  which  were  in  service  are  trunked  to  third  selectors  in  the 
offices  21,  22  and  23  whose  banks  are  multipled  with  the  existing  third 
selectors.  The  formerly  vacant  levels  (4  and  5)  are  trunked  to  third 
selectors  in  the  newly  opened  offices  (24  and  25). 

Sub-offices. — The  sub-office  is  a  very  simple  variation  in  the  general 
scheme  of  trunking.  Suppose  there  is,  at  some  distance  from  the  nearest 
office,  a  group  of  subscribers  such  that  the  cabling  of  these  lines  to  the 
office  is  a  matter  of  considerable  expense.  A  few  trunks  could  just  as 
well  handle  the  traffic  as  to  install  subscribers'  lines  complete,  running  the 
entire  distance.  To  handle  such  a  condition,  simply  remove  one  or 
more  lineswitch  units  from  the  office  and  locate  them  at  the  center  of 
distribution  to  this  group  of  subscribers.  We  may  imagine  the  trunk 
lines  connecting  this  unit  with  the  rest  of  the  exchange  to  be  stretched 


T RUN KING 


31 


out  to  reach  between  the  center  of  distribution  and  the  office.  Such  an 
installation  will  constitute  a  sub-office.  (Fig.  31.)  There  will  be  two 
kinds  of  trunks,  outgoing  trunks  from  the  sub-office  and  incoming  trunks 
to  it.  The  outgoing  trunks  run  from  the  banks  of  the  lines  witches  or 
the  wipers  of  finder  switches  and  terminate  in  first  selectors  in  the  main 
office.  The  incoming  trunks  run  from  the  banks  of  second  or  third 
selectors  (depending  upon  the  size  of  the  system)  and  terminate  in  jacks 
of  connector  switches  in  the  sub-office. 


,Sub-Offi'ce   | 


I    Branch  Office 
I     J sf:  Selectors 


Fig.  31. — Sub-office  trunking. 

In  order  to  reduce  the  number  of  trunk  wires  required  between  the 
banks  of  the  lineswitches  or  the  finder  switches  and  the  first  selectors  in 
the  main  office,  it  has  been  customary  to  install  a  repeater  in  the  sub- 
office  between  the  lineswitch  bank  and  each  outgoing  trunk.  This  also 
furnishes  a  convenient  point  for  the  supply  of  battery  current  to  the  call- 
ing subscriber,  because  the  greater  the  distance  over  which  battery  cur- 
rent must  be  supplied  the  weaker  will  be  the  transmission.  In  cases 
where  the  distance  from  the  district  sub-office  to  the  main  office  is  short 
enough  to  warrant  depending  upon  battery  supplied  from  the  main  office, 
the  "series"  type  of  repeater  or  trunk  holder  may  be  used.  This  trunk 
holder  has  the  simple  function  of  grounding  the  release  trunk  and  prevent- 
ing the  premature  release  of  the  lineswitch. 

Sub-office  with  Switching  Repeater. — In  the  ordinary  sub-office, 
every  local  connection  holds  two  trunks,  the  trunk  from  lineswitch  bank 
to  the  selector  in  the  main  office,  and  the  trunk  from  the  third  selector 
bank  in  the  main  office  to  the  connector  in  the  sub-office. 

This  can  be  avoided  by  using  the  switching  repeater.  (Fig.  32.) 
This  repeater  is  built  in  the  form  of  a  selector,  having  wipers,  banks, 


32 


A  UTOMA  TIC  TELEPHON  Y 


magnets,  and  relays.  There  is  one  of  these  per  trunk  from  the  line- 
switch  banks.  Each  such  trunk  also  terminates  in  a  secondary 
lineswitch.  The  trunks  to  the  main  office  start  at  these  secondary 
lines  witch  banks. 

The  levels  of  the  switching  repeater  lead  nowhere,  except  those  which 
correspond  to  the  division  in  which  the  sub-office  is  located.  A  "local" 
level  leads  to  local  connectors  in  the  sub-office. 

When  a  subscriber  in  the  sub-office  initiates  a  call,  the  primary  and 
secondary  lineswitches  extend  the  connection  to  a  first  selector  in  the 
larger  office. 


Pr.L.Sws. 


*-  c- 


Local  XJ 
Connectors* 


Inc. 


Local  V  I 
Connectors?- 
Inc>J 


Switching  Repeaters 


Jet  I  Sws. 


3 


scy 


:  To  First  Selectors 
in  Main  Office 


.  From  Selectors 
■  in  Main  Office 


Fig.  32. — Sub-office  with  switching  repeater. 


When  the  subscriber  dials  the  first  digit,  both  the  switching  repeater 
and  the  first  selector  lift  their  wipers  in  synchronism.  If  the  level 
called  leads  to  other  offices,  the  connection  is  completed  as  usual,  the 
repeater  acting  only  as  a  repeater. 

If  the  subscriber  dials  the  first  digit  of  his  own  office,  both  repeater  and 
selector  lift  their  wipers  in  synchronism,  when  the  repeater  rotates  into 
its  bank,  the  secondary  lineswitch  is  released  and  the  trunk  between 
offices  freed  to  be  used  by  others.  Succeeding  pulls  of  the  dial  complete 
the  connection  locally  in  an  obvious  manner.  A  1000-line  exchange  is 
assumed.     Any  number  of  hundreds  may  be  placed  in  the  sub-office. 

In  a  10,000-line  system,  the  repeater  which  switches  on  the  first 
digit  may  be  used,  but  second  selectors  must  be  interposed  between  the 
banks  of  the  repeaters  and  the  jacks  of  the  connectors.  (Fig.  33.)  This 
sets  aside  one  entire  block  of  1,000  numbers  for  the  sub-office,  none  of 
which  may  be  used  elsewhere.  When  a  subscriber  in  the  sub-office  dials 
a  number  in  that  office,  the  first  selector  in  the  main  office  goes  so  far 
as  to  seize  an  incoming  second  selector  in  the  sub-office,  but  both  first 


TRUNKING 


33 


and  second  selectors  are  released  when  the  repeater  switches  from  out- 
going trunk  to  wipers  and  local  second  selector. 

In  the  10,000-line  system  we  may  use  a  repeater  which  switches  on 
the  second  digit,  and  thus  do  away  with  local  second  selectors,  also 
permitting  some  of  the  hundreds  in  this  thousand  to  be  used  in  the  main 
office.     Figure  34  may  be  used  to  explain  this  trunking.     When  a  sub- 


Sub-Office 


Pr.LincSws. 


Switching  Repeaters 


1 

SecL.Sws.  \Mer-Officl  M 


scsvy 


i  Ma'm  Office 
1st  Selectors 


Connectorsr 


Fig.  33. — Sub-office  with  switching  repeater.  10,000-line  system  switch  on  first  digit. 

office  subscriber  calls  a  number  not  in  his  own  office,  the  wipers  of  the 
repeater  are  released  after  each  digit  and  no  local  connection  is  made. 
If  he  calls  a  number  in  the  sub-office,  the  wipers  are  released  after  the 
first  digit,  but  a  condition  is  created  in  the  repeater  so  that  when  the 
second  pull  of  the  dial  lifts  the  wipers  to  a  level  leading  to  local  connec- 
tors, the  repeater  rotates  the  wipers  to  hunt  an  idle  connector.  When 
the  connector  is  seized,  the  lines  are  switched  from  the  outgoing  trunk 


Sub-office 

Pr.UneSws. 

Swg.  Reps 


Sec ■  L.Sws 


Main  Office 
bt.Sels.  Bnd.Sels. 


Local 
Connectorj, 
Inc. 


Repeaters 
O 


Fig. 


34. — Sub-office  with  switching  repeater,  10,000-line  system  switch  on  second  digit. 


to  the  wipers  and  the  local  connector,  so  that  the  switches  in  the  main 
office  release. 

The  switching  depends  upon  both  of  the  first  two  digits.  Either  one 
being  wrong  will  cause  the  repeater  to  fail  to  switch  to  local. 

The  sub-office  may  be  increased  to  1000  lines,  maximum. 

The  same  switching  repeater  may  be  used  in  a  100,000-line  exchange 

3 


34  AUTOMATIC  TELEPHONY 


by  inserting  third  selectors  between  the  repeater  banks  and  the  local 
connectors.  The  sub-office  must  be  made  up  of  one  or  more  thousands, 
each  thousand  being  completely  set  aside  for  this  sub-office.  But  other 
thousands  in  the  same  10,000  may  be  used  elsewhere.  The  trunking 
limit  of  the  sub-office  is  10,000  lines,  but  this  is  never  reached,  because 
it  would  long  before  that  have  become  a  main  office  itself,  and  need  to  be 
equipped  with  first  selectors. 

In  the  100,000-line  system,  sub-offices  may  have  less  than  1000  lines 
each  by  using  a  switching  repeater  which  switches  on  the  third  digit. 
It  requires  no  third  selectors  in  the  sub-office,  because  the  bank  of  the 
repeater  is  connected  by  levels  to  the  connectors.  The  limit  is  1000 
lines  for  any  one  sub-office.  Any  hundred-line  unit  which  is  not  used  in 
a  sub-office  may  be  assigned  elsewhere. 

Two-way  trunks  between  a  sub-office  and  its  main  office  are  some- 
times used,  if  the  traffic  is  light,  or  has  its  busy-hour  load  one  way  at  a 
different  time  from  the  busy-hour  load  in  the  other  direction.     (Fig.  35.) 


k.  2300 


Main  Office 
Pr.LineSws.      Sub-Office  1st.  Selectors 

SecL.5tvs.  Inter -Office 


Switching  Repeaters        rpj      ' HHUtlL COR-3^,  =^TVT5=l ill  Incornm9 


Local 


Connectors  | 


Fig.  35. — Sub-office  with  2-way  trunks  and  switching  repeater. 


In  this  illustration  second  selectors  are  used  in  the  sub-office.  The 
incoming  second  selectors  differ  from  the  local  ones  in  having  a  switching 
or  cut-off  relay  to  switch  the  inter-office  trunk  from  the  selector  to  the 
secondary  lineswitch  for  an  outgoing  call.  The  incoming  first  selectors 
in  the  main  office  are  similarly  equipped. 

Normally  each  inter-office  trunk  ends  in  a  first  selector  in  the  main 
office  and  in  a  second  selector  in  the  sub-office.  When  a  sub-office  sub- 
scriber initiates  a  call,  the  secondary  lineswitch  takes  a  trunk  and,  over 
an  auxiliary  wire,  operates  the  cut-off  relay  (COR-1)  extending  the  con- 
nection to  the  first  selector  in  the  main  office.  Cut-off  relay  3  (COR-'S) 
is  prevented  from  operating,  but  the  first  selector  grounds  the  release 
trunk  through  the  repeater,  so  that  any  call  from  a  local  first  selector  will 
find  the  trunk  busy. 

When  a  main  office  subscriber  calls  the  sub-office,  the  first  selector 
(local)  takes  a  trunk  leading  through  a  repeater.  This  repeater,  over 
an  auxiliary  wire,  operates  the  cut-off  relay  (COR-3)  which  switches  the 
inter-office  trunk  onto  the  repeater.     At  the  sub-office,  the  cut-off  relay 


TRUNKING  35 

(COR-1)  does  not  operate,  but  the  incoming  selector  makes  the  trunk 
busy  so  that  no  secondary  lineswitch  can  take  it. 

If  two-way  trunks  are  used  with  a  sub-office  which  has  no  selectors 
but  has  more  than  100  lines,  the  inter-office  trunks  are  divided  into 
groups,  one  for  each  group  of  connectors  (100-line  unit).  For  traffic 
from  main  to  sub-office  they  operate  by  groups,  for  traffic  in  the  other 
direction  they  operate  as  one  group. 

In  the  foregoing  pages  of  this  chapter  the  practice  of  the  Automatic 
Electric  Company  has  been  described  quite  fully  in  order  to  give  the 
reader  a  clear  idea  of  the  general  principles  of  trunking  as  practised  with 
some  variations  by  all  of  the  companies  manufacturing  automatic  or 
semi-automatic  equipment.  Certain  of  the  variations  from  the  Auto- 
matic Electric  Company's  practice  have  been  noted,  so  that  the  following 
concise  statements  of  the  trunking  schemes  of  other  manufacturers  will 
complete  this  chapter. 

The  practices  of  the  Automatic  Telephone  Manufacturing  Company, 
of  Liverpool,  and  that  of  the  Compagnie  Francaise  pour  l'Exploitation  des 
Precedes  Thomson-Houston  of  Paris  are  the  same  as  those  of  the  Auto- 
matic Electric  Company. 

The  practice  of  the  Siemens  &  Halske  Company  is  the  same  as  that 
of  the  Automatic  Electric  Company,  except  that  it  employs  only  rotary 
lineswitches.  The  primary  and  secondary  lineswitcb.es  used  by  the  Sie- 
mens &  Halske  Company  in  full  automatic  systems  have  a  capacity  of 
ten  trunks  each,  while  in  its  traffic-distributor  system  a  similar  lineswitch 
having  a  capacity  of  25  trunks  is  used. 

In  the  American  Automatic  Telephone  Company  systems  finder 
switches  are  used  instead  of  lineswitches.  Each  finder  switch  has  a 
capacity  of  50  lines.  The  multiple  wiring  of  the  finder-switch  banks  is 
"slipped"  to  reduce  the  amount  of  wiper  movement  and  the  loss  of 
time  required  to  find  a  calling  line.  The  wipers  of  each  finder  switch 
are  the  terminals  of  a  first  selector  switch. 

The  wipers  of  these  first  selector  switches  do  not  have  both  a  vertical 
and  a  rotary  movement  as  in  the  Strowger  design,  but  have  a  rotary 
movement  only.  The  capacity  of  the  bank  is  50  trunks  in  ten  groups  of 
five  each,  and  the  rotary  movement  is  divided  into  two  actions.  The 
first  action  is  controlled  by  the  subscribers'  calling  device  and  rotates 
the  wipers  to  the  group  of  five  trunks  leading  to  the  section  of  the  switch- 
board in  which  is  terminated  the  line  of  the  party  being  called.  The 
second  action  is  entirely  automatic  and  places  the  wipers  on  the  terminals 
of  an  idle  one  of  the  five  trunks. 

The  second  selectors  act  the  same  as  the  first  selectors  and  the  banks 
of  the  connectors  are  the  same  as  those  of  the  first  selectors.  Of  course 
all  movements  of  the  connector  wipers  are  controlled  by  the  subscribers' 
calling  device.     Ordinarily,  five  connectors  are  put  in  one  group  but  by. 


36  AUTOMATIC  TELEPHONY 

the  use  of  "individuals"  more  are  sometimes  employed.  The  trunking 
scheme  of  this  company  is  more  fully  described  in  the  chapter  devoted 
to  its  apparatus. 

The  Clement  "Auto-manual"  system  uses  linefinder  switches  of  100 
lines  capacity.  These  switches  have  their  bank  contacts  arranged  simi- 
larly to  those  of  Strowger  switches,  but  the  contacts  are  set  on  edge  and 
placed  in  vertical  rows  instead  of  horizontal  rows.  The  wiper  rotates 
to  the  proper  row  and  then  moves  upward  to  the  proper  contact,  The 
contacts  are  set  on  edge  to  reduce  the  opportunity  for  dust  to  settle  on 
them. 

The  assignment  of  a  finder  switch  to  search  for  each  calling  line  is 
controlled  by  a  master  or  distributing  switch. 

As  in  the  American  Automatic  system,  each  finder  switch  is  tied 
"tail  to  tail"  to  a  first  selector  switch. 

The  first  selector,  second  selector  and  connector  switches  each  have 
100  capacity  banks  and  each  use  the  rotary  movement  first  and  vertical 
movement  second  as  the  finder  switches  do. 

When  a  calling  subscriber's  line  is  connected  to  an  idle  first  selector, 
another  idle  finder  switch  is  set  in  motion  and  bridges  a  second  trunk 
on  the  trunk  already  connecting  the  calling  party  to  a  first  selector  switch. 
This  bridged  trunk  leads  to  a  key-set  switch  which  operates  automatically 
and  extends  the  bridged  trunk  to  an  idle  set  of  calling  keys  in  some  opera- 
tor's position.  After  the  number  has  been  set  up  by  the  operator  and 
the  connection  has  been  completed,  the  key-set  switch  and  its  finder 
switch  automatically  release  and  await  another  call.  The  methods  em- 
ployed for  multipling  the  first  and  second  selectors  together  and  for  dis- 
tributing the  load  by  means  of  "slip"  wiring  are  the  same  as  those  used 
by  the  Automatic  Electric  Company. 

In  the  Western  Electric  Company's  European  system,  primary  and 
secondary  finder  switches  are  used.  These  switches  have  a  capacity  of 
60  lines  or  trunks  each.  Each  secondary  finder  is  tied  "tail  to  tail"  to  a 
first  selector  switch.  In  the  semi-automatic  system  of  this  company 
each  of  these  trunks  from  a  secondary  finder  switch  to  a  first  selector 
passes  through  an  operator's  position. 

In  either  system  the  first  selectors,  second  selectors  and  connectors 
have  200-point  banks.  The  selector  banks  are  arranged  in  ten  sets  of 
twenty  trunks  each.  The  lower  five  levels  of  the  second  selector  banks 
lead  to  connector  groups  in  one  thousand  and  the  upper  five  levels  to 
connector  groups  in  another  thousand.  It  is  obvious  that  since  each 
connector  switch  has  a  capacity  of  200  lines,  but  five  groups  of  such 
switches  are  required  for  completing  connections  to  1000  lines,  instead  of 
ten  groups  as  in  the  Strowger  system.  The  American  system  of  the 
Western  Electric  Company  is  described  in  detail  in  the  chapter  devoted 
to  that  company's  apparatus. 


T RUN KING  37 

In  the  Lorimer  system  finder  switches  were  used  which  have  access 
to  100  lines  each.  These  were  single-motion  switches  as  in  the  Western 
Electric  system.  Each  switch  had  ten  sets  of  wipers  which  were  selec- 
tively chosen.  Each  finder  was  tied  to  a  first  selector  and  each  first 
selector  had  as  many  second  selectors,  or  "interconnectors"  as  they 
are  called,  as  there  are  thousands.  The  connectors  had  a  capacity  of 
100  lines  each.     All  switches  had  a  rotary  wiper  movement  only. 


CHAPTER  III 

AUTOMATIC  ELECTRIC  COMPANY'S  APPARATUS 

The  systems  of  the  Automatic  Electric  Company  are  divided  into  the 
two  general  classes — "three-wire"  and  "two-wire."  The  term  "three- 
wire  is  commonly  applied  to  systems  which  require,  in  addition  to  the 
regular  metallic  circuit  from  a  telephone  to  its  central  office,  a  connection 
from  each  telephone  to  ground.  This  term  is  used  to  distinguish  these 
systems  from  what  are  called  "two-wire  systems"  and  which  require  no 
ground  connection  at  a  subscriber's  station. 

While  there  are  in  operation  a  large  number  of  three-wire  systems  of 
the  Automatic  Electric  Company's  manufacture,  which  are  either  of  the 
earlier  local  battery  type  (using  cells  of  battery  at  each  telephone  for 
supplying  current  for  talking  purposes)  or  else  of  the  later  common 
battery  type  and  which  are  giving  excellent  and  economical  service,  they 
are  thought  to  be  of  especial  interest  only  to  those  connected  with  the 
companies  operating  them  and  they  will,  therefore,  not  be  described  in 
this  chapter.  It  may  be  said,  however,  that  the  practice,  mechanisms  and 
principles  employed  in  these  systems  are  not  greatly  different  from  those 
used  in  the  two-wire  common  battery  systems  herein  described. 

The  Telephone  Instrument. — The  discussion  of  the  apparatus  will 
open  with  a  description  of  the  telephone  instrument,  with  special  reference 
to  the  automatic-calling  or  impulse-sending  device  used. 

Prior  to  the  year  1896,  an  automatic  telephone  subscriber  called  any 

number  which  he  might  desire  by 
pressing  push-buttons  on  his  tele- 
phone. There  were  generally  three 
push-buttons  arranged  and  labeled  as 
shown  in  Fig.  36.     If  the  subscriber 

Fig.  36. — Calling  push-buttons.  •  -i      j    ,  n    at        i  m     c  1 

wished  to  call  No.  143,  for  example, 
he  would  first  push  the  "hundreds"  button  once,  then  the  "tens"  button 
four  times,  and  finally  the  "units"  button  three  times.  While  this 
arrangement  gave  passable  service,  the  subscribers  made1  many  mistakes 
in  counting  the  pushes  and  sometimes  did  not  press  a  button  in  far 
enough,  or  hold  it  long  enough.  Consequently,  in  1896  a  contact -making 
machine  or  a  "calling  device,"  as  it  is  commonly  named,  was  substituted 
for  the  push-buttons.  A  wall  telephone  equipped  with  a  modern  calling 
device  is  shown  in  Fig.  37,  and  a  desk  telephone  in  Fig.  38. 

As  shown  in  these  figures  the  visible  portion  of  the  calling   device 

38 


AUTOMATIC  ELECTRIC  COMPANY'S  APPARATUS 


39 


Fig.  37. — Wall  telephone  instrument. 


consists  of  a  dial  pivoted  at  its  center,  so  that  it  may  be  turned  in  a  clock- 
wise direction.     For  convenience  in  turning  the  dial  it  has  finger  holes, 
ten  in  number,  around  its  outer  edge.     Through  each  finger  hole  a  num- 
ber is  seen ;  these  numbers  are 
consecutive  from  "1"  to  "9" 
and  through  the  tenth  finger 
hole,   "0"  appears.       In  the 
Automatic      Electric      Com- 
pany's  practice   "0"  always 
represents  "  10." 

To  call  "143,"  for  exam- 
ple,   a    subscriber    will    first 
remove  his  receiver  from  its 
switch    hook,    then    put    his 
finger  into  the  hole  through 
which  "1"  is  seen   and   pull 
the  dial  around  until  his  finger 
strikes    the    stop.       He    will 
then  take  out  his  finger,  allow- 
ing the  dial  to  return  to  nor- 
mal, and  place  it  in  the  hole  through  which  "4"  is  seen,  and  again  pull 
the  dial  round  until  his  finger  strikes  the  stop.     Finally,  he  places  his 
finger  in  the  hole  showing  "3,"  and  turns  the  dial  until  his  finger  again 
strikes  the  stop.     He  then  places  the  receiver  to  his  ear  and  awaits  the 

answer  of  the  party  called.  Each 
turn  of  the  dial  requires  approxi- 
mately one  second.  By  the  time  he 
has  placed  the  receiver  to  his  ear,  the 
'       r/,"iifrnni  automatic    machines    at  the    central 

office  will  have  completed  the  con- 
nection to  the  desired  line  and  will 
have  commenced  to  ring  intermit- 
tently the  bell  of  the  desired  tele- 
phone. If  the  number  called  by  a 
subscriber  is  busy,  his  receiver  will 
give  forth  an  intermittent  buzzing 
sound,  the  same  as  that  used  for  a 
busy  signal  in  large  manual  systems. 
When  through  talking  he  hangs  the 
receiver  on  the  switch  hook  and  the 
circuit  changes  thus  made  cause  the 
central  office  apparatus  to  return  to  normal  condition. 

There  is  contained  within  the  calling  device,  but  not  seen,  a  revolving 
cam,  arranged  to  make  and  break  the  contact  between  a  pair  of  springs. 


Fig.  38. — Desk  telephone  instrument. 


40 


A  UTOMA  TIC  TELEPHON  Y 


A  small  governor,  which  is  geared  to  the  cam  shaft,  controls  the  speed  at 
which  it  revolves.  The  power  is  furnished  by  a  piano-wire  spring  which 
is  rewound  each  time  the  subscriber  turns  the  dial.  The  cam  does  its 
work  after  the  subscriber's  finger  strikes  the  stop  and  as  the  dial  returns 
to  normal  position. 

A  photograph  of  the  calling-device  mechanism  which  is  ordinarily 
hidden  within  the  telephone  instrument  is  shown  in  Fig.  39,  and  a  drawing 

showing  the  functions  of  some  of  the 
parts  more  clearly  is  given  in  Fig.  40. 
As  the  dial  is  turned,  the  ratchet 
spring  snaps  from  tooth  to  tooth  of 
the  ratchet  wheel  and  at  the  same 
time  the  spring  coiled  around  the  dial 
shaft  (projecting  from  the  rear  of  the 
calling  device)  is  wound  up.  When 
the  dial  is  released  the  ratchet  spring 
at  once  engages  a  tooth  of  the  ratchet 
wheel  and  under  the  influence  of  the 
shaft  spring  the  dial  returns  to  its 
normal  position,  carrying  the  ratchet 
wheel  with  it.  The  governor  and 
impulse  cam  are  geared  to  the  ratchet 
wheel  and  are  therefore  operated  as 
the  dial  rotates  back  to  its  normal 
position.  Each  time  the  cam  re- 
volves, it  breaks  the  contact  between 
the  impulse  springs  twice. 
The  principles  involved  in  the  operation  of  the  dial  are  carefully 
worked  out  and  are  essential  to  rapid  and  accurate  calling.  Every  turn 
of  the  dial  is  positive  and  correct,  regardless  of  the  speed  at  which  it  is 
made.  Anyone  who  has  experienced  the  slow  and  painstaking  care 
required  to  manipulate  the  dial  of  an  ordinary  office  safe  to  bring  each 
successive  number  opposite  the  stopping  point  without  first  passing  it, 
will  readily  appreciate  that  any  calling  device  which  would  require  the 
subscriber  to  stop  each  number  opposite  a  pointer  or,  vice  versa,  to  stop 
a  pointer  opposite  each  number,  would  be  very  slow  and  inaccurate  in 
comparison  with  a  calling  device  like  that  shown  in  the  illustrations. 

The  only  feature  of  the  telephone  which  is  peculiar  to  automatic 
systems  is  the  calling  device.  The  signaling,  receiving  and  transmitting 
circuits  and  apparatus  may  be  the  same  as  those  used  in  any  common- 
battery  manual  telephone.  It  is  essential,  however,  that  the  circuits 
just  mentioned  be  connected  through  the  calling  device  in  such  a  way 
that  they,  will  be  automatically  disconnected  or  shunted  out  while  the 
calling  device  is  being  operated. 


Fig.  39. — Automatic  telephone  calling 
device. 


AUTOMATIC  ELECTRIC  COMPANY'S  APPARATUS 


41 


Telephone  Circuit  Diagram. — A  diagram  of  a  typical  circuit  is  shown 
in  Fig.  41.     When  the  receiver  is  removed  and  the  switch-hook  rises,  the 


r^^>%& 


^^^ 


iiiiiiiiii  i'i  i  i'iiiiiiiiiiii|l|||!l|TTf 
u*-K--,-M--v-K-v^^JJ 


bell  is  switched  out  of  circuit  and  the  transmitter,  receiver  and  impulse 
springs  are  connected  across  the  line. 

Each  time  the  impulse  cam  breaks  the  contact  between  the  impulse 
springs,  it  opens  the  line  for  an  instant,  thus  causing  one  "impulse"  to  be 


42 


A  UTOMA  TIC  TELEPHON  Y 


sent.  During  the  time  of  sending  impulses  the  receiver  and  transmitter 
are  both  shunted  out  of  the  circuit  by  the  action  of  the  shunt  springs, 
between  which  contact  is  closed  whenever  the  dial  is  turned  away  from  its 
normal  position.  This  contact  is  opened,  when  the  dial  returns  to 
normal,  by  the  bushing  at  the  end  of  the  arm  carried  by  the  dial  shaft. 

The  object  of  shunting  out  the  transmitter  and  receiver  during  im- 
pulse transmission  is  to  keep  the  resistance  of  the  subscriber's  loop  con- 
stant and  prevent  undue  noise  in  the  receiver.  Very  often  it  would  not 
remain  constant,  especially  on  a  desk  telephone,  if  the  transmitter  were 
in  circuit,  because  it  is  not  uncommon  for  a  subscriber  to  pick  up  a  tele- 
phone and  hold  it  at  various  angles  so  that  the  carbon  in  the  transmitter 


Bell 


Cond. 


fl 


P3E 


Impulse 
.-Springs 


■.[Shunt 
Spring 


0 
Cam  ^ 

s: 


Fig.  41. — Automatic  telephone  circuit,  series  type. 


shifts  about,  causing  wide  variations  of  the  transmitter  resistance  during 
the  time  the  dial  is  being  used.  Although  in  most  cases  the  operation 
would  be  satisfactory,  even  with  the  handicap  of  this  varied  resistance, 
it  is  safer  and  requires  less  careful  adjustment  of  the  mechanism  to  use 
the  shunt  springs.  Fully  to  comprehend  the  diagram  it  should  be  under- 
stood that  the  receiver  used  with  this  circuit  is  what  is  called  a  non- 
polarized direct-current  or  series  receiver,  i.e.,  it  has  no  permanent 
magnet,  but  uses  an  electromagnet  and  is  connected  in  series  with  the 
line  and  the  trasmitter  during  conversation. 

The  direct-current  type  of  receiver  is  not  essential  to  the  operation 
of  the  two-wire  system,  but  was  perfected  and  put  on  the  market  b}*  the 
Automatic  Electric  Company  at  about  the  time  their  two-wire  system 
was.  Since  it  is  especial^  applicable  to  that  system  but  is  not  applicable 
to  three-wire  systems,  it  has  become  identified  with  the  two-wire  systems. 
Several  of  the  first  two-wire  plants  installed,  however,  use  induction  coils 
and  receivers  with  permanent  magnets  at  the  subscribers'  stations. 

Varieties  of  telephone  instrument  circuits  will  be  found  in  Chapter 
on  "Subscribers'  Station  Equipment." 


AUTOMATIC  ELECTRIC  COMPANY'S  APPARATUS 


43 


In  Fig.  42  is  shown  an  ordinary  common-battery  manual  desk  tele- 
phone which  has  been  converted  into  an  automatic  instrument  by  mount- 
ing a  calling  device  upon  it  in  a  suitable  cup  clamped  to  the  handle  tube 
of  the  instrument.  This  indicates  one  of  the  attractive  features  of  this 
system,  which  is  that  almost  any  common-battery  telephone  may  be 
readily  converted  into  one  suitable  for  use  on  an  automatic  system  by 
mounting  a  calling  device  on  it  and  connecting  the  calling  device  in 
series  with  the  transmitter  of  the  instrument. 


Fig.  42. —  Manual  telephone  instrument  converted  into  an  automatic  instrument. 


Switchboard  Apparatus. — The  automatic  switchboards  used  in  sys- 
tems of  the  Automatic  Electric  Company  employ  the  following  principal 
pieces  of  apparatus: 

1.  Lineswitches  (primary  and  secondary)  with  their  master  switches. 

2.  Selector  switches  (first,  second  and  third). 

3.  Connector  switches. 

4.  Repeaters  and  trunk  holders. 

Whether  or  not  all  of  these  pieces  of  apparatus  are  used  in  any  system 
depends  upon  the  size  of  the  system  and  the  number  of  offices  in  it. 
Many  of  our  readers  are  more  familiar  with  manual  switchboard 


44  AUTOMATIC  TELEPHONY 

apparatus  than  with  the  principles  of  automatic  switchboards.  There- 
fore, this  explanation  of  automatic  switchboard  apparatus  will  begin 
with  a  description  of  the  two  pieces  of  equipment  whose  functions  are 
most  nearly  analogous  to  the  subscribers'  line  equipment  and  to  the  cord 
circuits  manipulated  by  an  operator  in  a  common-battery  manual  office. 

The  subscribers'  line  equipment  of  the  manual  is  resembled  by  the 
lineswitch  associated  with  each  line  of  the  automatic  switchboard;  and 
the  cord  circuit  manipulated  by  the  operator  is  resembled  by  the  connec- 
tor switch  of  the  automatic  switchboard.  By  means  of  these  two  pieces 
of  apparatus,  only,  a  system  of  not  more  than  100-lines  capacity  may  be 
built  up.  After  such  a  system  has  been  discussed,  the  equipment  used 
for  enlarging  it  into  a  system  of  almost  unlimited  capacity,  as  explained 
in  the  chapter  on  "Trunking,"  will  be  described. 

A  switchboard  for  100  lines  consists  of  100  lineswitches  with  their 
master  switch  and  other  associated  apparatus  and  about  ten  connector 
switches  for  making  interconnections  between  the  lines. 

The  Keith  Lineswitch. — The  Keith  lineswitch,  used  by  the  Automatic 
Electric  Company,  was  invented  by  Alexander  E.  Keith,  vice-president 
of  the  company.  It  is  distinct  from  those  of  the  rotary  type.  Essen- 
tial parts  of  each  lineswitch  are  its  line  relay  and  its  cut-off  relay.  The 
duties  of  the  latter  are  practically  the  same  as  those  of  the  relay  of  the 
same  name  associated  with  each  common-battery  manual  line,  while 
those  of  the  former  differ  in  one  important  respect,  viz.,  in  manual  prac- 
tice the  line  relay  operates  a  signal  to  attract  the  attention  of  an  operator 
who  connects  an  idle  cord  circuit  to  the  calling  line,  but  in  automatic 
practice  the  line  relay  causes  the  lineswitch  to  perform  the  reverse  func- 
tion of  connecting  the  calling  line  to  an  idle  cord  circuit  mechanism,  i.e., 
to  an  idle  connector  switch. 

The  duties  and  operation  of  Keith  lineswitches  have  been  explained 
at  length  in  the  chapter  on  trunking,  but  a  further  reference  to  them 
may  be  helpful  here.  Each  subscriber  is  equipped  with  a  lineswitch, 
consisting  of  a  magnet  and  a  lever,  carrying  on  its  end  a  plunger.  These 
plungers  are  maintained  in  alignment  by  means  of  the  guide  shaft,  whose 
edge  fits  into  a  notch  in  the  tail  of  the  plunger.  Immediately  in  front  of 
each  plunger  is  a  group  of  contacts  arranged  on  the  arc  of  a  circle.  Half 
of  these  contacts  are  attached  to  trunk  lines,  the  other  half  to  the  sub- 
scriber's line  to  which  this  lineswitch  belongs.  If  the  magnet  of  the  line- 
switch  be  energized,  the  plunger  will  be  driven  into  the  bank  and  by 
pressing  together  certain  springs  will  connect  the  subscriber's  line  to  a 
certain  one  of  the  trunk  lines.  These  trunk  lines  in  a  100-line  system 
lead  to  connector  switches  through  which  the  subscriber  obtains  access 
to  the  entire  exchange. 

The  master  switch  is  responsible  for  the  control  of  the  guide  shaft, 
as  indicated  in  Fig.  43.     The  means  of  connection  is  not  shown  in  this 


AUTOMATIC  ELECTRIC  COMPANY'S  APPARATUS 


45 


Master         g 
Magnet 


illustration.  The  duty  of  the  master  switch  may  be  divided  into  two 
parts : 

First. — The  duty  of  keeping  the  shaft  and  plungers  in  such  a  position 
that  all  the  plungers  will  be  pointing  toward  the  contacts  of  an  idle  trunk 
line. 

Second. — The  duty  of  preventing  any  lineswitch  from  operating  during 
the  time  that  the  master  switch  is  hunting 
an  idle  trunk. 

There  are  two  types  of  lineswitches 
used  in  two-wire  systems,  the  "single 
spool"  and  the  "two  spool."  Because 
the  single  spool  switch  has  not  been  made 
for  a  long  time,  only  the  two-spool  switch 
will  be  described. 

A  photograph  of  a  lineswitch  and  bank 
is  reproduced  in  Fig.  44.  The  line  relay 
is  at  the  left.  It  has  a  copper  collar  to 
make  it  slow  to  release.  The  plunger  is 
carried  by  the  plunger  arm,  under  which 
is  pivoted  the  cut-off  relay  armature.  The 
plunger  has  a  notch  in  its  fan-shaped  tail 
to  engage  the  guide  shaft.  The  point  of 
the  plunger  carries  two  little  hard  rubber 
rollers  to  press  the  bank  springs. 

Each  set  of  bank  contacts  consists  of 
four  movable  springs,  each  of  which  is 
pressed  against  a  contact  plate  when  the 
plunger  thrusts  its    roller   into  the  bank. 

Between  the  inner  two  rows  of  movable  bank  springs  is  a  "comb," 
between  the  teeth  of  which  the  point  of  the  plunger  passes,  so  that  the 
teeth  act  as  guides  for  the  plunger  movements. 

These  lineswitches  are  mounted  in  four  sets  of  25  each  on  an  "up- 
right" or  "unit"  as  shown  in  Fig.  86,  Chapter  4.  Each  set  of  25  has 
a  master  shaft.  Ordinarily,  the  four  master  shafts  are  linked  together 
and  controlled  by  one  master  switch,  but  in  some  cases  the  traffic  is 
so  heavy  that  it  is  necessary  to  provide  more  than  ten  trunks  for  100 
lines.  In  such  a  case,  the  lineswitches  are  divided  into  two  sets  of  50 
each  and  one  master  switch  is  mounted  for  controlling  the  two  guide 
shafts  of  each  set.  The  design  of  the  upright  contemplates  this,  for  the 
lineswitches  are  mounted  on  two  hinged  shelves,  each  of  which  carries 
one-half  of  the  switches  and  is  arranged  so  that  a  master  switch  may  be 
mounted  on  it  midway  between  its  top  and  bottom  and  between  its  two 
groups  of  25  lineswitches  each. 

The   lineswitch   unit   in   the   illustration   has   two  master  switches 


Fig.  43. — Scheme  of  master-switch 
control  of  Keith  lineswitches. 


46 


AUTOMATIC  TELEPHONY 


mounted  upon  it,  although  one  is  disconnected  and  the  master  shafts  are 
all  placed  under  the  control  of  the  other  by  means  of  the  connecting  rod 
with  a  turnbuckle  at  its  center  which  links  together  the  lower  ends  of 
the  upper  two  guide  shafts.  Any  lineswitch  may  be  readily  removed 
from  its  shelf  without  disconnecting  any  wires,  because  all  of  the  wiring 
of  each  switch  is  connected  to  a  set  of  jack  springs  as  shown  in  Fig.  44. 


Fig.  44. — Keith  lineswitch  with  an  unwired  bank. 


The  switch  jacks  engage  corresponding  shelf -jack  springs  as  shown  in 
Fig.  45,  which  gives  a  clearer  view  of  a  master  switch  and  a  few  mounted 
lineswitches.  The  lineswitches  in  this  illustration  are  of  the  two-spool 
type. 

Lineswitch  Circuits. — The  circuit  of  the  lineswitch  which  will  here 
be  discussed  has  its  duties  divided  into  two  parts : 

First. — Those  duties  which  must  be  performed  while  the  subscriber 
is  calling  into  the  exchange. 

Second. — Those  duties  to  be  performed  while  the  subscriber  is  being 
called  by  some  other  subscriber  through  a  connector  switch. 

The  duties  devolving  upon  the  lineswitch  circuit  while  calling  are  as 
follows: 

1.  Operate  the  plunger  to  extend  the  circuit  to  the  connector. 

2.  Clear  the  line  of  line  relay  and  ground  connections. 


AUTOMATIC  ELECTRIC  COMPANY'S  APPARATUS 


47 


3.  Drive  away  other  plungers  so  that  privacy  of  the  trunk  may  be 
secured. 

4.  Protect  the  subscriber's  line  at  the  connector  bank  so  that  no 
connector  can  seize  this  line. 

The  operation  of  the  lineswitch  circuit  (Fig.  46)  proceeds  as  follows: 
When  a  subscriber  lifts  his  receiver  from  the  switch  hook  the  circuit  is 
closed  from  earth  at  the  lineswitch  through  spring  (a)  to  the  positive 
side  of  line  to  the  telephone,  through  the  telephone  receiver  and  trans- 
mitter, back  on  the  negative  side  of  line  through  spring  (b)  and  the  line 
relay  winding,  L.R.,  of  the  lineswitch,  to  the  negative  pole  of  battery. 


Fig.  45. — Master  switch  mounted  on  shelf  of  Keith  lineswitch  unit. 


As  soon  as  this  circuit  is  established  the  line  relay  attracts  its  armature, 
closing  the  circuit  from  ground  through  the  "pull-down"  coil,  P.D.C., 
to  "open-main"  battery  bus  bar,  which  is  always  connected  to  the  nega- 
tive pole  of  battery  except  when  the  master  switch  is  moving  the  line- 
switch  plungers  from  one  trunk  to  another. 

When  the  "pull-down"  coil  is  energized,  it  attracts  the  bridge  cut-off 
relay,  B.C.O.,  armature,  which  disconnects  the  line-relay  coil  from  the 
line.  But  the  line-relay  armature  is  temporarily  retained  by  the  eddy 
current  in  the  copper  collar.  Following  the  quick-acting  cut-off  relay 
armature,  the  pull-down  coil,  P.  D.C.,  more  slowly  draws  down  the  plunger 
arm,  thus  thrusting  the  plunger  into  the  bank. 

When  the  plunger  is  thrust  into  the  bank,  circuit  is  closed  from  tele- 
phone to  the  line  relay  of  the  connector,  causing  it  indirectly  to  ground 
the  release  trunk.     This   energizes   the  B.C.O.,  which  holds  down  the 


IS 


AUTOMATIC  TELEPHONY 


bridge  cut-off  armature  and  also  holds  the  plunger  after  the  line-relay 
armature  has  dropped  back  and  opened  the  circuit  through  the  pull- 
down coil. 


-N+N  PN 


Fig.  46. — Circuit  of  Keith  lineswitch  and  master  switch. 

The  release  of  the  switch  is,  therefore,  controlled  through  the  release 
trunk  so  that  whenever  the  connection  between  the  release  trunk  and 
ground  is  broken  by  the  action  of  the  switch  ahead,  which  has  this  con- 
nection under  control,  the  bridge  cut-off  winding  of  the  lineswitch  will 


AUTOMATIC  ELECTRIC  COMPANY'S  APPARATUS  49 

release  the  plunger.  While  the  plunger  remains  in  the  bank  the  positive 
side  of  the  line  is  extended  through  to  the  trunk  by  contact  between 
bank  springs  7  and  8,  and  the  negative  line  by  contact  between  bank 
springs  5  and  6.  The  earth  connection  of  the  release  trunk  serves  also 
to  establish  a  guarding  potential  on  the  private  normal  P.N.  of  the  line- 
switch,  protecting  it  from  seizure  by  a  connector  switch  while  it  is  in  use. 
The  release  trunk  also  grounds  the  master  switch-bank  contact.  This 
causes  the  master  switch  to  operate  and  swing  the  plungers,  still  under 
the  control  of  its  shaft,  to  a  point  where  they  are  poised  over  the  bank 
springs  of  an  idle  trunk. 

The  springs  controlled  by  the  armature  of  the  bridge  cut-off  winding 
are  also  used  when  the  line  to  which  the  lineswitch  is  attached  is  called. 
At  such  a  time  the  bridge  cut-off  winding  is  energized  through  a  circuit 
that  will  be  explained  in  the  description  of  the  connector-switch  circuits, 
and  the  operation  of  these  springs  disconnects  the  line  relay  winding  and 
ground  of  the  lineswitch  but  does  not  affect  the  plunger  arm. 

Master  Switches. — The  master  switch  used  in  the  two-wire  system  is 
shown  in  Fig.  47,  and  its  circuits  are  given  in  Fig.  46. 

The  power  for  moving  the  master  switch  in  one  direction  is  supplied 
by  the  solenoid.  As  the  solenoid  draws  in  its  plunger,  it  not  only  moves 
the  master  shaft  but  it  also  draws  down  the  master-switch  17  spring  so  that 
when  the  master-switch  shaft  has  reached  the  limit  of  its  arc  of  movement 
in  one  direction,  the  master-switch  spring  is  ready  to  supply  the  power 
for  drawing  it  back  to  the  other  end  of  the  arc.  The  speed  with  which  the 
master  shaft  moves  is  controlled  by  the  governor,  which  is  of  the  same 
type  as  that  used  in  the  calling  devices. 

When  the  lineswitch  whose  action  we  are  discussing  seizes  a  trunk, 
the  ground  placed  on  the  corresponding  master-bank  contact  energizes 
the  starting  relay  of  the  master  switch.  The  illustration  shows  the 
master  switch  as  holding  the  guide  shaft  so  as  to  point  the  plungers  at 
the  tenth  trunk.  The  starting  relay  causes  the  lock  magnet  to  pull  up, 
withdrawing  the  lock  lever  so  that  the  escapement  releases  the  sector. 
The  force  of  the  U-shaped  spring  moves  the  guide  shaft  with  the  idle 
plungers.  The  governor  restrains  the  speed.  During  this  time  the 
lock  magnet  has  energized  the  open  main  relay,  which  cuts  negative 
battery  from  the  pull-down  coils  so  that  no  subscriber  can  plunge  in  his 
switch. 

When  the  master-switch  wiper  strikes  an  ungrounded  contact,  indi- 
cating a  free  trunk,  the  starting  relay  falls  back  and  releases  the  lock 
magnet.  Instantly  the  lock  lever  drops  the  roller  into  a  notch  in  the 
sector,  stopping  the  guide  shaft  with  the  idle  plungers  poised  opposite 
the  idle  trunk.  The  open  main  relay  now  releases  and  closes  the  nega- 
tive battery  wire  to  the  pull-down  coils. 

When  the  guide  shaft  has  reached  trunk  No.  1,  the  U-spring  has  done 


50 


A  I  'TOM A  TIC   TELEPHONY 


all  that  it  can  do.  As  soon  as  a  subscriber  takes  this  trunk,  the  sole- 
noid magnet  pulls  the  shaft  back  to  trunk  No.  10,  ready  to  start  all  over 
again.     The  way  that  it  is  done  is  this: 

When  the  guide  shaft  is  opposite  trunk  No.  1,  finger  No.  1  is  pressing 
together  the  two-finger  springs.  When  the  starting  relay  operates,  the 
trip  relay  is  pulled  up  as  well  as  the  lock  magnet.  The  springs  of  the 
trip  relay  are  caught  by  the  latch  and  held  together.  These  springs  keep 
the  lock  magnet  energized  regardless  of  the  starting  relay.  They  also 
energize  the  solenoid  magnet.     The  latter  pulls  the  guide  shaft  to  trunk 


Fig.  47. — Unmounted  master  switch. 


No.  10,  where  the  finger  No.  10  trips  open  the  springs  of  the  trip  relay. 
This  cuts  off  the  current  from  the  solenoid  magnet,  and  returns  the  lock 
magnet  to  the  control  of  the  starting  relay.  From  then  on,  the  action 
is  as  has  been  described.  If  the  tenth  trunk  is  free,  the  shaft  will  stay 
there.  If  it  is  busy,  the  starting  relay  will  cause  the  hunt  to  begin,  the 
shaft  being  moved  by  the  U-spring. 

The  several  symbols  marked  " Supy"  refer  to  a  common  relay  which 
gives  switchroom  supervision.  The  details  of  this  will  be  described  in 
the  chapter  on  Power  Plant. 

The  condenser  placed  in  parallel  with  the  solenoid  is  to  reduce  the 
arc  at  the  contacts  of  the  trip  relay.  A  similar  condenser  is  provided 
for  the  lock  magnet. 


AUTOMATIC  ELECTRIC  COMPANY'S  APPARATUS 


51 


Rotary  Lineswitch. — This  is  a  later  development  which  avoids  com- 
mon mechanism  (master  switch)  and  permits  much  larger  trunk  groups 
and  greater  flexibility  of  arrangement.  While  rotary  lineswitchcs  with 
ten  trunks  (Siemens  and  Halske)  have  been  made,  the  Automatic  Elec- 
tric Company  made  its  bank  with  25  sets  of  contacts.  It  was  also 
arranged  so  that  50  sets  of  contacts  can  be  used  by  the  same  mechanism. 
The  advantages  of  large  trunk  groups  are  discussed  elsewhere. 

The  rotary  lineswitch,  Fig.  48,  has  a 
bank  and  four  double-ended  wipers,  so 
that  oach  rotation  through  a  half  circle 
runs  a  wiper  over  all  trunks.  The  cur- 
rent is  carried  to  each  wiper  by  two 
springs.  The  private  wiper,  by  means 
of  which  the  trunks  are  hunted,  has  a 
longer  trailing  tip  than  the  rest. 

The  motor  magnet  drives  the  wipers 
on  its  return  stroke.  The  pawl  rests 
normally  between  the  toothed  wheel 
and  a  stationary  lug,  locking  the  wipers 
in  position.  The  wipers  rest  wherever 
they  were  when  last  used,  and  never 
move  unless  it  is  necessary  to  hunt  an 
idle  trunk. 

There  is  a  line  relay  and  a  cut-off 
relay  with  each  lineswitch.  The  former 
is  slow-releasing,  the  latter  is  quick-act- 
ing.    There  is  a  mechanical  interlocking 

device   which    permits    the    CUt-off    relay    Fig.  48.— Rotary  lineswitch  with  bank. 

to  move  its  armature  only  part  way  if 

the  line  relay  is  normal.  This  breaks  all  back  contacts,  but  does  not 
close  any  front  contacts.  If  the  line  relay  is  energized,  the  cut-off  relay 
can  move  its  armature  a  full  stroke. 

The  circuits  of  the  rotary  lineswitch,  Fig.  49,  are  very  simple.  The 
line  relay  is  normally  connected  to  the  subscriber  line  with  the  usual 
polarity  of  battery.  The  cut-off  relay  is  in  series  with  the  motor  magnet 
and  the  lead  to  the  private  normal  PN.  The  line  relay  initially  protects 
the  calling  line  from  intrusion  and  connects  the  trunk-hunting  circuit. 
The  cut-off  relay  siezes  the  free  trunk  and  extends  the  connection. 

When  the  subscriber  takes  his  receiver  from  the  hook,  the  line  relay 
energizes,  grounds  the  private  normal  PN  for  protection,  connects  the 
motor  magnet  MM  to  the  private  wiper  to  test  the  trunk,  and  removes 
the  mechanical  lock  from  the  cut-off  relay  armature.  In  the  large 
majority  of  cases  the  trunk  is  free.  The  magnet,  finding  no  direct  ground, 
will  not  act  through  the  high  resistance  of  the  cut-off  relay  COR.     The 


52 


A  I  TOM  A  TH  ■   TELEPHONY 


cut-off  relay,  drawing  current  through  the  magnet  and  springs  3-4  of 
the  line  relay,  pulls  up,  switching  both  subscriber  lines  —  L  +  L  from 
ground,  line  relay  and  battery  to  the  wipers  —  W  +  W.  It  also  switches 
the  private  wiper  PW  from  the  test  circuit  of  springs  1-2  of  the  line  relay 
LR  to  the  private  normal  and  the  cut-off  relay. 

The  extension  of  the  subscriber  lines  to  the  connector  causes  its  line 
relay  and  release  relay  Rise  Ry  to  ground  the  release  trunk.  This  holds 
the  cut-off  relay  of  the  lineswitch  energized  and  keeps  the  private  normal 
grounded.  The  line  relay  of  the  lineswitch,  being  slow  to  release,  only 
now  falls  back. 

Ro+aru  L'me switch 


Fig.  49. — Circuit  of  rotary  lineswitch.      Pencil  sketch. 


During  conversation  the  line  circuit  runs  through  the  lineswitch  with- 
out attachments  of  any  kind.  The  cut-off  relay  is  the  only  energized 
coil,  drawing  its  weak  current  through  the  motor  magnet. 

Release  is  accomplished  by  removing  ground  from  the  release  trunk. 
The  cut-off  relay  simply  releases  and  re-establishes  normal  conditions. 

If  the  trunk  on  which  the  wipers  were  resting  at  the  start  is  busy, 
there  is  a  ground  on  the  private  contact  and  hence  on  the  private  wiper 
PW.  This  permits  the  motor  magnet  to  get  a  direct  ground.  The  cut- 
off relay  is  also  short  circuited  because  there  is  a  ground  at  both  ends  of 
its  winding.  The  magnet  pulls  up  and  its  pawl  catches  the  next  tooth 
on  the  wheel.  Near  the  end  of  its  stroke  the  magnet  opens  its  own  cir- 
cuit (like  a  door  bell)  so  that  the  spring  can  drive  the  pawl  back,  rotating 
the  wipers  to  the  next  trunk  contacts.  If  this  trunk  also  is  busy,  the 
performance  is  repeated.  The  cut-off  relay  can  not  energize  because  of 
the  short-circuit. 

When  the  private  wiper  strikes  an  ungrounded  contact,  the  magnet 
receives  current  only  through  the  cut-off  relay,  which  is  too  little  to 
actuate  the  magnet.  But  the  cut-off  relay  promptly  acts,  siezing  the 
trunk  as  was  described  above. 

The  condenser  and  small  resistance  attached  to  the  magnet  are  to 
reduce  the  arc  at  its  own  interrupter  contact. 

When  this  line  is  called  by  some  other  line,  the  call  comes  through 
a  connector  switch  and  the  normal  wires.     The  connector  first  tests  the 


AUTOMATIC  ELECTRIC  COMPANY'S  APPARATUS 


53 


private  normal  PN  to  see  if  the  line  is  busy  or  free.  If  busy,  the  private 
normal  is  grounded,  and  the  connector  can  not  sieze  the  line.  If  the 
private  normal  is  not  grounded,  the  connector  first  puts  a  ground  on  it. 
This  is  to  keep  other  connectors  away  and  to  operate  the  cut-off  relay 
to  clear  the  line  of  attachments.  Since  the  line  relay  is  normal,  the  cut- 
off relay  can  pull  its  armature  up  only  part  way.  It  is  enough  to  break 
all  back  contacts,  cutting  off  the  ground,  line  relay  and  battery,  and  to 
disconnect  the  private  wiper.  The  connector  delays  ringing  until  this 
has  been  accomplished. 

The  Connector  Switch. — As  already  stated, 
the  functions  of  the  connector  switch  re- 
semble those  of  a  manual  operator  and  her 
equipment. 

This  switch  is  able  to  connect  a  calling 
party's  line  with  any  one  of  the  100  subscribers 
whose  lines  are  terminated  in  its  banks.  It 
makes  the  busy  test  and  switches  the  busy 
signal  on  to  the  calling  party's  line  or  ring- 
ing current  on  to  the  called  party's  line  as 
may  be  required.  It  supplies  the  current  for 
energizing  the  transmitters.  When  conversa  ■ 
tion  is  completed  it  controls  circuits  which 
cause  all  of  the  switches  used  in  setting  up 
the  connection  to  return  to  normal. 

The  banks  of  this  switch  correspond  to  the 
line  jacks  before  an  operator  in  a  manual 
board  equipped  for  100  lines  only.  The  shaft 
of  the  switch  corresponds  to  the  operator's 
hand  and  arm  and  the  shaft  wipers  to  the 
plug  which  the  operator's  hand  and  arm 
manipulates.  The  wiper  cords  take  the  place 
of  the  switchboard  cord.  In  other  words,  this 
switch  is  an  automatically  operated  and  controlled  cord  circuit. 

Figure  59  may  be  used  as  the  basis  of  the  following  discussion: 

Mechanism. — Figure  50  is  from  a  photograph  of  one  type  of  two-wire 
connector  switch  with  a  set  of  unwired  banks.  In  the  front  of  the  switch 
is  seen  the  shaft  which  carries  the  contact  arms  or  wipers,  and  which  can 
be  raised  step-by-step  by  a  pawl  and  ratchet  movement  operated  by  the 
vertical  magnet,  and  can  be  rotated  step-by-step  through  about  120 
degrees  by  another  pawl  and  ratchet  movement  operated  by  the  rotary 
magnet.  The  vertical  movement  is  opposed  by  gravity  and  the  rotary 
movement  by  a  clock  spring  carried  in  a  small  box  or  cup  at  the  top  of  the 
shaft.  At  the  completion  of  each  vertical  or  rotary  step,  the  shaft  is 
locked  in  position  by  the  "double  dog."     When  the  release  of  a  connec- 


Fig.  50. — Connector  switch. 


.-.I 


A  I  TO  MA  TIC   TELEPHONY 


tion  occurs,  the  release  armature  pushes  the  double  dog  out  of  engagement 
with  the  shaft  and  into  engagement  with  a  catch  called  the  "release  link," 
which  holds  it  until  another  call  is  initiated.  When  the  double  dog 
disengages  it,  the  shaft  rotates  back  to  its  original  angle,  under  the 
influence  of  the  clock  spring  and  there  drops  down 
to  its  normal  position  by  the  force  of  gravity. 
The  drawing  in  Fig.  54  of  the  shaft  with  its 
wipers  shows  more  clearly  the  method  of  in- 
sulating each  of  the  wipers  from  the  shaft,  and 
the  springs  of  the  line  wipers  from  each  other. 
At  the  upper  end  of  the  shaft  is  a  set  of  springs 
known  as  the  off-normal  springs  and   indicated 


Fig.   51. — Automatic  relay  mounting,  top  view. 


Fig.  52. — Back  of  con- 
nector switch. 


by  the  designation  O.N .S.  When  the  shaft  is  in  its  normal  position  the 
springs  are  in  one  condition.  When  the  shaft  is  lifted  one  step,  the  finger 
connected  to  the  shaft  releases  its  pressure  on  the  off-normal  springs  and 
allows  them  to  perform  whatever  switching  action  they  are  designed  to  do. 


Fig.   53. — Switch  shelf,  showing  jacks. 


In  Fig.  50,  the  upper  bank  is  the  private  bank,  while  the  lower  one  is 
the  line  bank.  In  each  bank  the  rows  number  "1"  to  "9"  then  "0," 
in  regular  order  counting  from  the  bottom  row  to  the  top.  Each  wiper 
is  so  placed  on  the  shaft  that  when  it  is  raised  one  step,  it  will  be  on  a 


AUTOMATIC  ELECTRIC  COMPANY'S  APPARATUS 


55 


3X3 


— w 


level  with  row  1  of  its  bank  and  the  next  step  will  place  it  on  a  level  with 
row  2,  etc. 

For   convenience  all  of  the  relays  forming  a  part  of  the  connector- 
switch  circuit  are  mounted  on  the  connector-switch 
frame,  although  they  are  not  a  part  of  its  mechanism 
and   could  be  mounted  elsewhere.     A  top  view  of 
two  relays  is  given  in  Fig.  51. 

All  the  wiring  leading  from  the  switch  magnets 
or  the  relays  terminates  the  spring  of  the  "jack," 
shown  at  the  rear  of  the  switch  (Fig.  52).  When  the 
switch  is  hung  on  its  shelf  these  jack  springs  engage 
those  of  the  shelf  jack  (Fig.  53). 

In  Fig.  55  is  shown  an  unwired  private  bank, 
while  Fig.  56  is  a  sectional  view  showing  how  the 
brass  contact  pieces  are  laid  between  strips  of  insula- 
tion to  form  the  assembly.  Figure  57  illustrates  the 
looping  of  the  wires  from  bank  to  bank  in  a  set  of 
line  banks.  The  twisted  pair  of  wires  that  is  soldered 
to  pair  01  on  the  bank  at  the  left  loops  over  and  is 
soldered  on  the  corresponding  pair  in  each  of  the 
other  banks.  In  this  way  all  pairs  of  the  same 
number  are  multipled  together. 

A  feature  of  the  connector  relays,  which  should 
be  understood,  before  discussing  the  circuit,  is  the 
use  of  slow-acting  coils.     Such  a  coil  is  contrived 
by  placing  a  heavy  ring  or  cylinder  of  very  pure  copper  over  one  end 
of  the  core  of  the  spool,  occupying  a  portion  of  the  space  used  for  the 


Fig. 


54. — Switch   shaft 
and  wipers. 


«  *  g  gj 

-  r  ■ 

-ts/r 

#      ft 

THfefc 

r    «s 

rtfiBflBBJBUhi- 

<*  yj£& 

•'^yj 

L 

%JF 

Fig.  55. — Connector-switch  bank,  unwired. 


winding  on  an  ordinary  spool.  The  outside  diameter  of  this  ring,  which 
has  a  rectangular  cross  section,  is  the  same  as  that  of  the  spool  head,  and 
the  inside  diameter  is  just  large  enough  to  allow  the  ring  to  fit  snugly  over 


56 


A  UTOMA  TIC  TELEPHON  Y 


the  core.  (See  Fig.  58.)  A  relay  of  this  type  attracts  its  armature 
nearly  as  quickly  as  an  ordinary  relay,  but  when  Ihe  circuit  through  its 
winding  is  broken  the  current  induced  in  the  heavy  copper  ring  retains 


Fig.  56. — Detail  of  connector-switch  bank. 


the  armature  for  the  fraction  of  a  second,  giving  it  a  very  noticeable  lag. 
As  explained  more  fully  in  the  description  of  the  circuits  of  this  system, 


Fig.   57. — Wiring  of  connector-switch  banks. 


the  introduction  of  these  slow-acting  relays  makes  possible  the  operation 
of  the  switches  by  simply  making  and  breaking  the  calling  party's  loop, 


r*.   ^y 


Quick  Acting 


Slow  Acting         -fj" 
Collar 


Fig.   58. — Relays,  quick  and  slow. 


and  the  consequent  elimination  of  the  ground  required  at  each  subscriber's 
station  in  the  three-wire  system 

Circuits  of  Switches. — While  describing  the  electrical  circuits  of  the 
connectors  and  other  switches,  the  reader  is  asked  to  keep  in  mind  that 


AUTOMATIC  ELECTRIC  COMPANY'S  APPARATUS  57 

circuits  like  machines  are  made  up  of  elementary  parts,  each  of  which  is 
exceedingly  simple.  These  elementary  parts  may  be  termed  "circuit 
devices."  Upon  the  uses  to  which  a  switch  is  put  depends  the  choice  of 
circuit  devices  and  the  manner  in  which  they  are  combined.  We  will, 
therefore,  first  consider  the  duties  which  devolve  upon  each  switch  and 
then  explain  the  circuit  devices  and  combinations  by  which  the  desired 
results  are  obtained. 

Connector  Circuits. — In  order  that  the  reader  may  have  already  in 
mind  the  functions  of  the  connector  circuits,  the  duties  of  the  switch  are 
recapitulated  as  follows: 

1.  Execute  the  vertical  and  rotary  movement  of  the  shaft  under  the 
control  of  the  calling  device. 

2.  Keep  the  wipers  disconnected  during  rotation  so  as  not  to  interfere 
with  lines  past  which  they  rotate. 

3.  Test  the  called  line  to  see  if  it  is  busy.  This  busy  test  consists  of 
two  parts:  (a)  A  lock  to  prevent  intrusion  on  to  the  busy  line,  (6)  a  signal 
to  notify  the  calling  station  that  the  line  is  busy. 

4.  Protect  the  called  line  from  intrusion. 

5.  Clear  the  called  line  of  attachments. 

6.  Ring  the  bell  at  the  called  station. 

7.  Supply  battery  current  to  both  stations  for  conversation. 

8.  Release  itself  without  interfering  with  other  lines. 

9.  Release  other  switches  (selectors,  etc.). 

In  Fig.  59  is  shown  a  skeleton  drawing  of  the  mechanism.  This  will 
help  to  make  clear  the  operation  of  the  switch  as  the  circuits  are  discussed. 

The  calling-line  circuit  (Fig.  60)  leads  from  negative  line  —  L  and 
positive  line  +L  through  the  line  relay  LR  to  battery,  48  volts.  The 
normal  polarity  is  as  drawn,  but  it  is  reversed  by  the  back-bridge  relay 
BBR  which  feeds  the  called  line.  Condensers  connect  the  two  current 
supplies. 

The  line  relay  LR  controls  the  release  relay  RlRy  and  repeats  line 
impulses  to  the  magnets,  VM  and  RM.  The  release  relay  grounds 
the  release  trunk,  holds  open  the  release  magnet  circuit  Rise,  prepares 
the  impulse  circuit  for  the  magnets,  and  furnishes  ground  for  the  locking 
of  the  ring  cut-off  relay  RCOR  and  the  wiper-closing  relay  WCRy. 

The  series  relay  Se  co-operates  with  the  vertical  magnet  as  the  rotary 
relay  RotRy  does  with  the  rotary  magnet  RM.  The  busy-test  relay  ByRy 
tests  the  called  line,  furnishing  busy  tone  and  preventing  the  wiper- 
closing  relay  from  connecting  the  wipers.  The  wiper-closing  relay 
WCRy  connects  the  line  circuit  to  the  wipers,  starts  the  ringing  inter- 
rupter, Ri.  Int.,  grounds  the  private  wiper  PW ,  prevents  further  rotation, 
and  cuts  off  the  busy-test  relay.  The  ring  cut-off  relay  RCOR  cuts  off 
the  ringing  current  when  the  called  station  answers,  and  furnishes 
positive  battery  to  the  back-bridge  relay  BBR. 


58 


A  UTOMA  Tl(  •  TELEPHON  V 


When  the  lineswitch  connects  a  calling  subscriber  line  to  a  connector, 
the  line  relay  of  the  connector  immediately  pulls  up  and  energizes  the 
release  relay.  The  latter  grounds  the  release  trunk  so  that  the  lineswitch 
will  not  release.  This  also  makes  the  connector  busy  so  that  no  other 
lineswitch  will  seize  it. 


Off  Normal 
Springs 


Stationery 
Dog 


Interrupter 
Springs 


Cam 


Line  Wiper 


O  ]^  Vertical 
Magnet 


Rotary 
Magnet 


Line  Bank 


Fig.   59. — Switch  mechanism. 


When  the  subscriber  dials  the  first  digit  of  the  call  number,  the  line- 
relay  armature  vibrates  one  complete  cycle  for  each  unit  in  the  digit. 
Thus  it  feeds  current  alternately  to  the  release  relay  and  to  the  vertical 
magnet.  The  former  is  slow  release,  and  remains  energized  throughout 
the  series  of  pulsations  which  it  receives.  The  vertical  magnet  is  quick- 
acting,  follows  the  impulses  and  lifts  the  wiper  shaft.     The  series  relay  Se 


AUTOMATIC  ELECTRIC  COMPANY'S  APPARATUS 


59 


energizes  with  the  first  impulse  and  remains  energized  to  the  end  of  the 
series  of  pulsations  fed  to  the  vertical  magnet. 

The  first  upward  step  of  the  wiper  shaft  operates  the  off-normal 
springs  ONS.  These  springs  prepare  the  release  circuit  (for  the  release 
magnet  Rise),  but  action  is  prevented  by  the  release  relay  (back  contact 
open).  The  off-normal  springs  also  switch  the  impulse  circuit  of  the 
vertical  magnet  from  the  winding  of  the  series  relay  to  the  main  spring 
of  the  same.  But  since  this  relay  is  energized,  the  vertical  magnet  con- 
tinues to  get  the  impulses. 

When  the  impulses  cease,  the  series  relay  falls  back  and  switches  the 
impulse  circuit  to  the  rotary  magnet  RM  and  rotary  relay  RotRy.  A 
pause  of  a  little  less  than  a  second  is  sufficient  for  the  change  to  take 
place. 


Rise  Trk. 


Ri.  Int.  Start 
Fig.   60. — Circuit  of  connector  switch. 


When  the  subscriber  dials  the  second  digit  of  the  call  number,  the 
line  relay  again  vibrates,  but  this  time  the  pulsations  act  on  the  rotary 
magnet  and  the  rotary  relay  RotRy  in  parallel.  The  rotary  magnet  ro- 
tates the  wipers  over  the  bank.  The  line  wipers  are  open,  but  the  private 
wiper  PW  carries  the  test  circuit  which  comes  from  negative  battery 
through  the  winding  of  the  busy-test  relay  ByRy. 

As  the  wipers  progress  toward  the  desired  line,  the  private  wiper 
may  pass  over  one  or  more  busy  lines.  Their  private  contacts  are 
grounded.  Hence  the  busy  relay  will  be  energized  while  passing  over 
such  contacts.  This  would  cut  off  the  rotary  magnet,  if  it  were  not  for 
the  rotary  relay,  which  maintains  the  circuit  for  the  rotary  magnet  until 
the  group  of  pulsations  has  ceased. 

When  the  group  of  impulses  ceases,  the  rotary  relay  de-energizes. 
Assume  that  the  line  is  not  busy.  The  busy-test  relay  reveals  this  con- 
dition by  being  de-energized.     There  is  therefore  a  circuit  from  ground 


60  AUTOMATIC  TELEPHONY 

at  the  release  relay,  through  a  back  contact  on  the  busy  relay,  125-ohm 
winding  of  the  wiper-closing  relay,  back  contact  of  the  rotary  relay,  to 
the  private  wiper.  This  circuit  is  completed  through  the  cut-off  relay 
winding  of  the  lineswitch  of  the  called  line  to  negative  battery.  Simul- 
taneously the  latter  relay  clears  the  called  line  while  the  former  closes 
the  lire  wipers  to  the  source  of  ringing  current,  which  is  controlled  bjr  the 
ring  cut  off  relay.  The  wiper-closing  relay  also  grounds  the  private  wiper 
with  a  direct  ground  and  starts  the  ringing  interrupter  Ri.  Int.  It  is 
itself  locked  by  its  1300-ohm  winding  to  a  ground  at  the  release  relay. 
This  contact  is  the  first  to  close. 

Ringing  takes  place  intermittently,  by  the  action  of  the  ringing 
interrupter.  The  wire  through  which  ringing  current  is  delivered  is 
switched  alternately  between  the  ringing  generator  and  ground  (positive 
battery).  The  complete  circuit  of  the  ringing  current  includes  the  200- 
ohm  winding  of  the  ring  cut-off  relay  and  the  main  battery,  48  volts. 
During  the  ringing  period,  the  battery  is  unable  to  affect  the  ring  cut-off 
relay  because  of  the  condenser  in  series  with  the  bell  in  the  telephone  set. 
The  ringing  current  can  not  operate  the  relay,  because  it  is  equipped  with 
a  copper  collar  and  a  copper  sleeve,  the  latter  under  the  winding.  De- 
pendence is  placed  upon  the  battery  alone  to  operate  the  relay. 

If  the  called  subscriber  answers  during  the  ringing  period  or  between 
rings,  the  battery  current  energizes  the  200-ohm  winding  of  the  ring 
cut-off  relay.  The  contact  for  the  locking  winding,  1300  ohms,  closes 
first.  The  relay  switches  the  line  wipers  from  the  ringing  generator  to 
the  talking  circuit  and  opens  the  ringing  interrupter  start  wire.  The 
locking  contact  also  supplies  positive  battery  to  the  back-bridge  relay 
BBR. 

The  switching  of  the  lines  to  the  talking  circuit  causes  the  called 
station  to  draw  battery  current  through  the  back-bridge  relay,  which 
energizes  and  reverses  the  polarity  of  current  supplied  to  the  calling 
subscriber. 

The  subscribers  may  now  talk,  under  the  conditions  shown  in  Fig. 
61.  The  reversal  of  battery  current  is  used  to  operate  service  meters, 
give  supervision  to  a  manual  operator,  etc.  These  uses  will  be  described 
later. 

The  release  of  the  connection  is  controlled  by  the  calling  party.  Sup- 
pose that  the  called  party  hangs  up  his  receiver  first.  The  back-bridge 
relay  de-energizes,  but  nothing  else  happens,  because  the  line  relay  and 
the  release  relay  hold  the  release-magnet  circuit  open.  When  the  calling 
subscriber  hangs  up,  the  line  relay  falls  back,  followed  by  the  release 
relajr.  The  latter  takes  the  ground  off  the  release  trunk,  permits  the 
lineswitch  to  release,  and  also  takes  the  locking  ground  off  the  ring  cut-off 
relay  and  the  wiper-closing  relay.  The  latter  releases  quickly,  to  open 
the  line  wipers  and  to  take  the  ground  off  the  private  wiper.     The  release 


AUTOMATIC  ELECTRIC  COMPANY'S  APPARATUS 


61 


magnet  gets  current  through  the  back  contact  of  the  line  relay,  back 
contact  of  release  relay,  back  contact  of  back-bridge  relay,  and  the  off- 
normal  springs.  It  pulls  the  double  dog  from  the  shaft,  which  now 
restores  to  normal.  When  the  shaft  finishes  its  drop,  the  off -normal 
springs  return  to  normal,  opening  the  release-magnet  circuit. 

The  ring  cut-off  relay  is  slower  to  release  than  the  wiper-closing 
relay,  so  that  the  line  wipers  may  have  no  ringing  current  on  them  while 
they  are  rotating  back  off  the  bank.  It  would  annoy  other  lines  which 
might  be  in  use. 

If  the  calling  subscriber  hangs  up  first,  the  lineswitch  will  release,  but 
the  connector  will  not.  The  line  relay  falls  back  and  energizes  the  rotary 
relay  before  the  release  relay  has  time  to  let  go.  Then  the  release  relay 
falls  back,  so  that  for  a  brief  time  there  is  no  ground  on  the  release  trunk. 
Yet  the  release-magnet  circuit  of  the  connector  is  open,  being  held  so 
by  the  back-bridge  relay.  Then  the  rotary  relay  releases  and  replaces 
the  ground  on  the  release  trunk,  so  that  another  lineswitch  may  not 
seize  this  connector.     This  ground  is  through  125  ohms,  but  it  is  effective. 


Calling  Telephone  Line 

0=» 


Line 


Called 
Telephone 


2ME 
Fig.   61. — Talking  conditions  in  single-office  exchange. 


When  the  called  subscriber  hangs  up,  the  back-bridge  relay  closes 
the  release-magnet  circuit  and  takes  the  locking  ground  off  the  ring  cut- 
off relay  and  the  wiper-closing  relay.  The  release  of  the  connector  pro- 
ceeds as  described  above. 

The  holding  of  the  connector  until  the  last  party  has  hung  up  prevents 
undue  noise  in  the  ear  of  the  called  party  when  the  calling  party  hangs 
up  first.  It  also  keeps  the  called  subscriber  from  seizing  another  switch, 
which  will  happen  if  he  stays  on  the  line  after  the  connector  has  been 
released. 

The  contact  on  the  back-bridge  relay  marked  "  S-2"  is  provided 
to  give  switch-room  supervision.  A  lamp  connected  to  it  will  glow  if  the 
called  station  holds  on  after  the  calling  subscriber  has  released.  The 
contact  marked  "S-l"  is  not  often  provided.  It  may  be  used  to  give 
switch-room  supervision  by  a  lamp  if  the  calling  subscriber  hangs  onto 
the  line  after  the  called  subscriber  has  hung  up. 

The  condenser  and  small  resistance  attached  to  the  front  contact  of 
the  release  relay  is  to  reduce  the  spark  at  the  back  contact  of  the  line 


62 


A  VTOMA  TI( '  TELEPIION  Y 


relay  during  dialing.     The  inductance  of  the  vertical  magnet  and  later 

of  the  rotary  magnet  causes  the  arc. 

The  energy  from  the  magnetic  field  of  the  release  magnet  is  absorbed 

by  the  500-ohm  non-inductive  resistance  in  parallel  with  the  100-ohm 

winding.     It  greatly  relieves  the  contact  of  the  off-normal  springs. 

Selector   Switches. — The   uses   of   first,    second   and   third   selector 

switches  are  explained  at  length  in  the  chapter  on  trunking. 

Since  a  selector  is  used  for  selecting  trunks  only,  its  circuit  is  much 

simpler  than  that  of  the  connector.     The  switch  mechanism  proper  is 

practically  the  same.     A  photograph  of  a  selector  switch  with  a  set  of 

unwired  banks  mounted  upon  it  is  repro- 
duced in  Fig.  62.  It  employs  the  same 
type  of  shaft,  wipers,  banks,  vertical  and 
rotary  ratchet  movements,  release  mechan- 
ism, etc.,  as  the  connector  switch  does. 
One  essential  difference,  however,  is  that 
the  vertical  movement  only  is  controlled 
from  the  calling  party's  calling  device. 
The  rotary  movement  is  entirely  auto- 
matic and  is  used  to  select  an  idle  trunk 
from  the  set  of  trunks  represented  by  the 
row  of  bank  contacts  to  which  the  wipers 
have  been  directed  in  the  vertical  move- 
ment of  the  switch  shaft.  This  is  fully 
discussed  in  the  chapter  on  trunking. 

Briefly,  the  duties  of  a  selector  may  be 
summarized  as  follows: 

1.  To    perform   the    vertical    motion 
under  the  control  of  the  calling  device. 

2.  To  hunt  and  seize  an  idle  trunk, 
(a)   Rotate   automatically   without 

regard  to  the  calling  device. 
(6)  Keep  line  wipers  clear. 

(c)  Test  each  trunk. 

(d)  Stop  rotary  motion  at  first  idle  trunk. 

(e)  Protect  seized  trunk. 

(/)    Extend  line  circuit  through  to  next  switch  without  attachments. 

3.  Release  itself  without  interfering  with  other  trunks. 

A  diagram  of  a  circuit  of  a  first  selector  is  shown  in  Fig.  63.  The  two 
line  wires  normally  run  to  the  line  relay  LR,  but  are  capable  of  being 
switched  to  the  line  wipers,  —W  and  -\-W.  The  functions  of  the  line 
relay  are  to  energize  the  release  relay  RlRy,  to  repeat  impulses  to  the  ver- 
tical magnet  VM,  and  to  control  the  circuit  of  the  release  magnet  Rise 
if  it  is  desired  to  release  the  selector  before  it  has  completed  its  work. 


Fig.   62. — Selector  switch. 


AUTOMATIC  ELECTRIC  COMPANY'S  APPARATUS 


63 


The  release  relay  places  a  ground  on  the  release  trunk  and  holds  it 
there  during  the  operation  of  the  switch  and  until  the  next  switch  ahead 
can  take  up  that  duty.  The  release  trunk  will  be  connected  straight 
through  the  selector  when  the  lines  are  extended.  The  release  relay 
helps  to  control  the  release-magnet  circuit  and  prepares  the  impulsing 
circuit  from  the  line  relay  to  the  vertical  magnet. 

The  series  relay  Se  co-operates  with  the  off-normal  springs  ONS  to 
initiate  the  rotary  magnet  RM  action  when  the  vertical  magnet  has  done 
its  work. 

The  rotary-interrupter  relay  RIR  tests  the  trunks  and  helps  the  rotary 
magnet  to  vibrate  in  driving  the  wiper  shaft  around. 


Rise.  Trk. 


M.Ry 


Fig.   63. — Circuit  of  selector  switch. 

The  switching  relay  SwRy  switches  the  lines  from  the  line  relay  to 
the  line  wipers  —  W  and  +  W,  extends  the  release  trunk  to  the  next  switch 
ahead  and  controls  the  ground  connection  of  the  line  relay  so  as  to  take 
charge  of  the  release  of  the  switch. 

Normally  the  positive  battery  connection  of  the  line  relay  has  in 
it  a  coil  which  furnishes  a  tone  to  indicate  to  a  subscriber  that  a  selector 
is  ready  for  use.     It  is  called  the  "trunk  tone"  or  "dial  tone." 

There  is  an  auxiliary  set  of  springs  marked  "11-R"  which  come  into 
play  when  the  entire  ten  trunks  of  a  level  are  busy  and  the  wipers  there- 
fore rotate  off  the  bank.  These  springs  open  the  circuit  of  the  switching 
relay  to  prevent  its  action  and  substitute  the  regular  busy  tone  on  the 
line  relay,  so  that  the  subscriber  will  be  induced  to  hang  up  and  release. 

The  action  of  the  switch  is  as  follows.  When  a  selector  is  seized  by 
a  lineswitch  or  by  another  selector,  the  subscriber  line  or  its  equivalent 


64  AUTOMATIC  TELEPHONY 

is  connected  to  the  line  wires,  —  L  and  -\-L,  so  that  the  line  relay  at  once 
energizes  and  causes  the  release  relay  to  pull  up.  The  latter  places  a 
ground  on  the  release  trunk,  so  that  all  switches  back  of  it  will  be  held 
up.  At  the  same  time  it  prepares  the  impulse  circuit  to  the  verticil 
magnet. 

When  the  subscriber  pulls  the  dial  for  the  digit  which  is  to  operate 
this  switch,  the  line  relay  vibrates  in  unison  with  the  line  impulsi  3. 
This  keeps  the  slow-releasing  release  relay  energized  and  at  the  same  time 
sends  pulsations  to  the  vertical  magnet  and  the  series  relay.  The  latter 
pulls  up  and  remains  energized,  the  former  follows  the  impulses  and  lifts 
the  wiper  shaft. 

The  first  upward  step  of  the  shaft  operates  the  off-normal  springs. 
e  pair  of  springs  prepares  the  release-magnet  circuit.     The  other  pair 

inects  the  rotary-interrupter  relay  to  the  front  contact  of  the  series 

v.  As  the  series  relay  is  energized,  the  rotary-interrupter  relay  also 
pulls  up  by  reason  of  the  ground  on  the  release  trunk.  It  locks  itself 
pcrgized  by  a  circuit  which  runs  through  the  contact  of  the  rotary  mag- 
net to  the  ground  on  a  spring  of  the  switching  relay. 

When  the  series  of  impulses  end,  the  line  relay  comes  to  rest  energized, 
holding  the  release  relay.  The  series  relay  falls  back,  closing  the  cir- 
cuit of  the  rotary  magnet  from  the  release  trunk  ground  through  a  front 
contact  on  the  rotary-interrupter  relay.  The  rotary-interrupter  relay 
now  depends  upon  the  ground  at  the  switching  relay,  while  the  rotary 
magnet  depends  on  the  release  trunk. 

The  rotary  magnet  now  pulls  up,  moving  the  wipers  onto  the  first 
set  of  trunk  contacts.  If  this  trunk  is  free,  there  will  be  no  ground  on  its 
private  contact.  If  it  is  busy,  there  will  be  a  ground  there.  Assume  the 
latter  to  be  the  case.  This  ground  on  the  private  bank  contact  is  in 
effect  on  the  private  wiper  and  passes  through  a  back  contact  on  the 
switching  relay  to  the  spring  of  the  rotary  magnet  and  of  the  interrupter 
relay. 

Near  the  end  of  its  stroke,  the  rotary  magnet  opens  its  springs, 
cutting  off  the  interrupter  relay.  The  latter  falls  away,  opening  the 
rotary-magnet  circuit.  The  rotary  magnet  falls  back,  closing  the  cir- 
cuit of  the  interrupter  relay  to  the  private  wiper.  At  this  time  the 
interrupter  relay  depends  solely  upon  the  private  wiper  for  a  circuit, 
because  its  own  circuit  to  the  ground  at  the  switching  relay  is  open. 
If  the  private  wiper  again  rests  on  a  grounded  contact,  the  interrupter 
relay  will  pull  up  again  and  give  current  to  the  rotary  magnet,  causing 
it  to  rotate  the  wipers  to  the  next  trunk.  As  long  as  the  private  wiper 
finds  ground,  the  interrupter  relay  and  the  rotary  magnet  will  vibrate. 

During  this  time  the  switching  relay  has  had  a  ground  at  each  end 
of  its  winding,  therefore  it  could  not  act. 

When  the  private  wiper  strikes  an  ungrounded  contact,  the  rotary- 


.       AUTOMATIC  ELECTRIC  COMPANY'S  APPARATUS  65 

interrupter  relay  fails  to  pull  up.  The  switching  relay  immediately 
energizes,  getting  its  positive  battery  from  the  release  trunk  ground  at 
the  release  relay  and  its  negative  battery  from  the  circuit  through  the 
rotary-magnet  contact  and  the  interrupter  relay  winding. 

The  pulling  up  of  the  switching  relay  cuts  off  the  line  relay  and  ex- 
tends the  lines  to  the  next  switch  ahead.  It  removes  the  ground  from 
the  line-relay  main  spring  so  that  when  that  relay  de-energizes  it  will  not 
actuate  the  vertical  magnet.  This  same  contact  also  controls  the  re- 
lease magnet.  The  switching  relay  also  closes  the  release  trunk  through 
to  the  private  wiper. 

Before  the  release  relay  of  this  selector  can  fall  back,  the  line  relay 
and  release  relay  of  the  next  switch  ahead  has  grounded  its  release  trunk, 
so  that  the  private  wiper  of  this  selector  now  has  ground  on  it.  Then 
the  release  relay  in  this  selector  lets  go,  but  the  continuity  of  ground  on 
the  release  trunk  is  maintained. 

The  switching  relay  alone  remains  energized.  It  is  the  holding  relay 
for  this  selector. 

Release  from  a  completed  connection  is  accomplished  by  removing 
the  ground  from  the  release  trunk.  The  switching  relay  falls  back, 
opens  the  line  wipers,  and  closes  the  release-magnet  circuit.  The  release 
magnet  presses  the  double  dog  out  of  engagement  with  the  shaft,  allow- 
ing the  latter  to  rotate  and  drop  to  normal.  As  it  reaches  the  bottom, 
it  operates  the  off-normal  springs,  opening  the  release-magnet  circuit 
and  that  of  the  interrupter  relay. 

If  all  trunks  on  the  level  are  busy,  the  private  wiper  will  be  unable 
to  find  absence  of  ground.  The  wipers  will  therefore  rotate  off  the  bank. 
This  is  called  the  eleventh  position.  Here  the  11-R  springs  act,  cutting 
off  the  switching  relay  and  putting  on  the  busy  tone.  The  subscriber, 
hearing  the  tone,  will  release.  The  line  relay  falls  back  and  gives  a 
momentary  pulsation  to  the  vertical  magnet,  after  which  the  release  relay 
falls  back  and  energizes  the  release  magnet.  The  rest  is  as  before.  This 
case  does  not  occur  very  often  and  is  not  detrimental. 

Ten-Level  20-Trunk  Selector. — The  trunking  capacity  of  this  type  of 
selector  has  been  increased  by  equipping  it  with  two  line  ban'ks,  making 
its  private  bank  with  the  contacts  in  pairs,  and  arranging  the  circuit  so 
that  as  the  wipers  rotate  over  both  line  banks  at  the  same  time  they  hunt 
idle  trunks  and  seize  the  first  one  found,  whether  it  be  in  the  upper  bank 
or  the  lower  one.  If  the  two  sets  of  wipers  find  an  idle  trunk  at  the  same 
time,  one  only  is  seized,  with  the  certainty  that  the  other  is  not  touched. 

The  circuit  (Fig.  64)  has  the  usual  line  relay,  release  relay,  series 
relay,  release  magnet,  vertical  magnet,  etc.,  as  in  other  selectors. 

There  are  two  switching  relays,  SR-1  and  SR-2.  The  former  switches 
the  lines  from  the  line  relay  to  line  wipers  —Wl  and  +  IP1.  The  latter 
switches  the  lines  from  the  line  relay  to  the  line  wipers  —  W2  and  +  W2. 

5 


60 


A  V  TOM  A  TIC  TELEPHON  Y 


Each  of  the  switching  relays  also  takes  part  in  the  trunk  seizure;  in  almost 
the  same  way  as  does  the  switching  relay  of  the  ordinary  selector. 

The  rotary  trunk-hunting  action  consists  of  mutual  vibration  between 
the  rotary  magnet  RM  and  the  rotary-interrupter  relay  RIR.  The  latter 
is  double-wound  and  is  so  adjusted  that  it  will  not  move  its  armature 
unless  both  windings  are  energized,  and  if  while  energized  one  of  the 
windings  fails,  the  armature  will  be  released. 

The  switch  is  seized  in  the  usual  way,  the  line  relay  and  the  release 
relay  pulling  up  and  grounding  the  release  trunk.  The  group  of  line 
impulses  cause  the  line  relay  to  vibrate,  during  which  time  the  vertical 
magnet  lifts  the  shaft  and  the  series  relay  Se  is  continuously  energized. 
The  latter  pulls  up  the  rotary -interrupter  relay  RIR,  which  locks  itself 
by  two  contacts  on  to  the  ground  furnished  by  the  two  switching  relays. 


Rise.  Trk. 


Fig.  64. — Circuit  of   10-levcl  20-trunk  selector. 


The  rotary-interrupter  relay  prepares  a  circuit  for  the  rotary  magnet, 
which  is  yet  open  at  the  contacts  of  the  series  relay. 

When  the  group  of  impulses  comes  to  an  end,  the  line  relay  stops  in 
an  energized  condition,  the  release  relay  as  well.  Then  the  series  relay 
falls  back,  closing  the  circuit  for  the  rotary  magnet  to  the  release  trunk 
extension.  The  rotary  magnet  now  rotates  the  wipers  onto  the  first 
set  of  contacts  of  the  level  selected.  Assume  that  the  first  few  trunks 
are  busy  on  both  line  banks. 

Since  both  private  wipers  are  resting  on  ground,  the  switching  relays 
can  not  pull  up.  Near  the  end  of  its  stroke,  the  rotary  magnet  opens 
the  circuit  of  the  rotary-interrupter  relay  RIR,  which  falls  back.  As  it 
releases,  it  opens  its  locking  circuits  and  cuts  off  the  rotary  magnet.  The 
rotary  magnet  falls  back  and  closes  the  circuit  of  the  interrupter  relay. 
If  both  private  wipers  rest  on  ground,  the  interrupter  relay  will  energize, 


AUTOMATIC  ELECTRIC  COMPANY'S  APPARATUS  67 

lock  itself,  and  energize  the  rotary  magnet.  In  this  way  the  mutual 
vibration  of  magnet  and  relay  continue  as  long  as  both  private  wipers 
can  find  ground. 

When  one  of  the  private  wipers  finds  a  free  trunk,  there  will  be  no 
ground  between  a  winding  of  the  interrupter  relay  and  the  corresponding 
switching  relay.  Assume  that  it  is  P-l  which  finds  no  ground.  At  the 
moment  that  the  rotary  magnet  closes  the  circuit  by  falling  back,  there 
is  no  ground  between  the  left-hand  winding  of  RIR  and  the  winding  of 
SR-l.  The  former  can  not  energize  through  the  latter.  But  the  SR-1 
will  pull  up  at  once,  switch  P-l  from  RIR  so  as  to  extend  the  release 
trunk  through  to  the  bank,  switch  the  lines  from  line  relay  to  the  line 
wipers  on  line  bank  No.  1  and  cut  off  the  ground  from  LR  and  RIR  so 
that  it  can  do  no  harm.  It  also  cuts  off  the  other  relay  SR-2,  so  that  it 
can  not  operate.  Soon  the  line  relay  and  release  relay  of  the  next  switch 
ahead  will  ground  the  extended  release  trunk  and  hold  the  connection. 

If  both  private  wipers  find  a  free  trunk  at  the  same  time,  there  is  a 
tendency  for  SR-1  and  SR-2  to  operate  together.  But  each  is  wired  so 
as  to  cut  off  the  other.  Therefore  that  relay  which  acts  more  quickly 
will  cut  off  the  other  and  take  the  trunk.  It  is  impossible  for  both  to 
act  together.  This  circuit  device  has  been  known  for  many  years  and 
is  a  very  delicate  test  for  the  speed  of  two  relays. 

This  switch  circuit  shows  the  dial  tone  normally  on  the  line  relay 
with  the  busy  tone  ready  to  be  substituted  if  all  trunks  on  the  level  are 
busy  and  the  wipers  rotate  off  the  bank  and  operate  the  springs  "  ll-R." 
The  same  springs  open  the  release  trunk  extension  to  prevent  the  switch- 
ing relays  from  operating. 

Multi-Level  Group  Selector. — It  sometimes  happens  that  a  small 
exchange  or  a  small  office  in  a  multi-office  exchange  has  need  for  more 
than  one  level  of  trunks  to  a  certain  destination  and  can  spare  the  call 
numbers  which  the  additional  levels  represent.  In  this  case  a  selector 
may  be  arranged  to  treat  several  levels  as  in  one  group. 

The  multi-level  group  selector  responds  to  the  calling  device,  lifting 
the  wipers  to  the  desired  level  in  the  same  way  as  is  done  by  an  ordinary 
selector.  If  there  is  an  idle  trunk  on  the  first  or  bottom  level  of  the 
group,  the  wipers  will  rotate  onto  that  level  and  hunt  an  idle  trunk. 
But  if  the  first  level  is  busy,  the  wipers  will  be  lifted  to  the  next  level, 
its  condition  tested,  and  the  wipers  rotated  into  the  bank  or  lifted  to  the 
next  level  as  the  test  may  direct. 

A  selector  of  this  type  requires  horizontal-chain  relays,  one  for  each 
trunk  in  a  level,  vertical  chain  relays,  one  for  each  level,  and  a  vertical 
wiper  and  bank.     (See  Fig.  65  and  66.) 

When  a  trunk  is  occupied,  its  horizontal-chain  relay  pulls  up.  When 
all  the  trunks  in  the  same  level  are  busy,  the  chain  of  contacts  is  com- 
pleted and  the   corresponding  vertical-chain  relay  is  energized.     This 


68 


A  U  TOM  A  TIC  TELEPHON  Y 


relay  places  a  ground  on  the  corresponding  contact  in  the  vertical  bank, 
which  contact  the  vertical  wiper  will  engage  when  the  line  wipers  stand 
opposite  the  level  which  is  busy. 

The  vertical  bank  contact  for  the  upper  level  of  a  group  is  open. 

The  vertical  wiper  has  connected  to  it  a  testing  circuit  which  runs  to 
negative  battery  through  the  vertical  relay  which  controls  the  vertical 
runk  hunting. 


Fig.  65. — Vertical  wiper  and  bank,  normal. 


When  the  switch  is  seized,  the  line  relay  and  release  relay  operate  as 
usual,  grounding  the  release  trunk  and  preparing  the  vertical-magnet 
circuit. 

The  calling  device  of  the  telephone  makes  the  line  relay  vibrate  and 
thus  to  cause  the  vertical  magnet  to  lift  the  shaft  to  the  selected  level. 

If  the  calling  device  stops  the  vertical  motion  with  the  vertical  wiper 
resting  on  a  grounded  contact,  the  vertical  relay  will  energize. 

The  vertical  magnet  at  once  operates  again,  and  will  continue  to  do  so 
as  long  as  the  vertical  magnet  finds  ground,  this  operation  results  in  the 
lifting  the  wiper  shaft  while  the  levels  are  tested  one  by  one. 

When  the  vertical  relay  finds  no  ground  it  will  fall  back,  at  this  mo- 


AUTOMATIC  ELECTRIC  COMPANY'S  APPARATUS 


69 


ment,  the  interrupter  relay  is  de-energized  and  the  vertical  motion  ceases. 
Following  this  the  wipers  are  moved  onto  the  first  trunk  of  the 
selected  level. 

As  long  as  the  private  wiper  finds  grounded  contacts,  the  mutual 
vibration  of  the  rotary  magnet  and  the  interrupter  relay  will  cause 
continued  rotation  of  the  wipers. 

When  an  idle  trunk  is  found,  the  absence  of  ground  on  the  private 
wiper  will  prevent  the  interrupter  from  acting  again.  The  switching 
relay  will  now  operate  in  the  usual  manner. 


Fig.  66. — Vortical  wiper  and  bank,  operated. 


The  next  switch  ahead  soon  grounds  the  release  trunk,  so  that  when 
the  line  relay  and  release  relay  of  this  selector  fall  back,  the  switching 
relay  will  be  held  energized. 

If  there  is  a  free  trunk  in  the  first  level  of  the  group,  the  rotary  action 
starts  at  once. 

If  all  levels  are  busy,  the  switch  will  rotate  on  the  upper  level  of  the 
group  anyway,  because  the  corresponding  vertical-bank  contact  is  open. 
Since  the  upper  level  is  busy,  the  same  as  the  rest,  the  usual  rotary  action 


70 


A  UTOMA  TIC  TELEPHON  Y 


will  carry  the  wipers  off  the  bank  and  operate  a  set  of  cam  springs. 
They  open  the  switching  relay  winding  so  as  to  prevent  its  action,  and 

Release  Trunk A  .      ■ — 1)|. 

-T 


Chain  Rly 
Contacts 

Fig.   67. — Circuit  of  impulse  repeater. 

switch  the  ground  connection  of  the  line  relay  from  dial  tone  to  busy 

tone. 

Repeater  Circuits. — Automatic  impulse 
repeaters  are  introduced  in  outgoing  trunks 
from  one  office  to  another.     (See   Fig.    68.) 

r-i ''■^         SP^^  <        ''"'  PurPoses  °f  ;l  repeater  are  as  follows: 
-*^«*^^  '  1.  Ground  the  release  trunk  to  permit  the 

use  of  two-wire  trunks  between  offices. 

2.  Supply  talking  current  to  the  calling 
station  from  the  home  office. 

3.  Repeat  impulses  to  the  distant  office. 
A  typical  repeater  circuit  is  given  in  Fig. 

67. 

Three  wires  arrive  from  the  left,  release 
trunk,  negative  line  —  L  and  positive  line  +L. 
^SF        Two  wires  deparl  to  the  right,  negativetrunk  — 
^m^m  T  and  positive  trunk  +  T.     The  line  circuit 

has  the  line  relay  LR  with  battery  to  supply 
current,  and  the  trunk  circuit  has  the  back- 
bridge  relay  BBR  and  the  chain  relay  ChRij 
as  a  bridge.     The  two  are  connected  by  con- 
densers which  are  normally  2  MF. 
The  line  relay  LR  repeats  the  impulses  from  the  line  to  the  trunk.     It 
also  controls  the  release  relay  RlRy  and  the  shunting  relay  ShRy.     The 
release  relay  grounds  the  release  trunk,  prepares  the  circuit  for  the  shunt- 


2 


Fig.   68. — Impulse  repeater. 


AUTOMATIC  ELECTRIC  COMPANY'S  APPARATUS  71 

ing  relay,  and  polarizes  the  back-bridge  relay.  These  two  relays  (LR 
and  RlRy)  have  in  general  the  same  duties  and  relationships  which 
they  will  be  found  to  have  in  very  many  pieces  of  apparatus. 

The  shunting  relay  ShRy  cuts  out  the  back-bridge  circuit  during  dialing 
and  assists  the  chain  relay  ChRy  in  its  duties.  The  latter  is  chiefly  an 
impedance,  but  also  acts  as  a  relay  in  co-operation  with  similar  relays  of 
other  repeaters  to  indicate  when  all  trunks  in  a  group  are  busy.  Details 
will  be  given  later. 

The  back-bridge  relay  BBR  causes  the  reversal  of  the  current  fed  to 
the  calling  line,  so  as  to  repeat  the  supervision  sent  back  by  the  con- 
nector in  the  distant  office.  Since  the  load  of  springs  is  great,  the  work 
itself  is  not  put  on  the  back-bridge  relay,  but  on  the  reversing  relay  RevRy, 
which  the  former  merely  controls.  The  back-bridge  relay  is  electrically 
polarized  by  its  1900-ohm  winding,  so  that  it  will  not  operate  when  the 
trunk  current  is  normal,  but  will  pull  up  and  remain  energized  thereafter. 
The  polarizing  winding  is  not  able  to  operate  it  alone. 

The  reversing  relay,  in  addition  to  reversing  the  current,  increases  the 
impedance  of  the  chain  relay  by  cutting  in  additional  turns. 

When  a  selector  seizes  a  trunk  in  which  is  this  repeater,  the  extension 
of  the  subscriber  line  causes  the  line  relay  of  the  repeater  to  pull  up. 
The  release  relay  therefore  also  energizes.  These  two  relays  close  the 
trunk  circuit,  so  that  the  line  relay  of  the  selector  in  the  distant  office  will 
come  into  action.  The  chain  relay  in  the  repeater  will  close  its  contacts. 
The  release  relay  grounds  the  release  trunk  so  that  the  selector  and  the 
lineswitch  will  be  held  up. 

When  the  subscriber  dials  a  digit,  the  line  relay  of  the  repeater  vibrates 
as  described  in  connection  with  other  apparatus.  The  first  time  that  it 
falls  back,  it  energizes  the  shunting  relay,  which  opens  the  negative 
trunk  —  T  to  the  back  bridge  and  connects  it  directly  across  the  line  to  the 
main  spring  of  the  line  relay.  The  rest  of  the  impulses  travel  over  this 
short  circuit  without  having  to  overcome  the  impedance  of  the  relays 
in  the  back  bridge.  The  line  relay  keeps  both  the  release  relay  and 
the  shunting  relay  energized  during  the  group  of  impulses. 

At  the  end  of  the  group  of  impulses,  the  line  relay  comes  to  rest 
energized.  The  release  relay  remains  up,  but  the  shunting  relay  falls 
back,  restoring  the  back  bridge  to  the  trunk  circuit.  This  action  is 
repeated  every  time  a  digit  is  pulled. 

When  the  called  station  in  the  distant  office  answers,  the  connector 
reverses  the  battery  current  as  was  described.  The  change  of  direction 
of  current  through  the  60-ohm  winding  of  the  back-bridge  relay  causes 
it  to  assist  the  polarizing  winding,  and  operates  the  contacts.  One  pair 
of  springs  shunts  the  impulse  springs  of  the  line  relay,  so  that  this  contact 
shall  be  assured  even  if  the  calling  loop  is  long.  It  also  operates  the 
reversing  relay  to  reverse  the  current  flow  in  the  calling  line  and  increase 
the  impedance  of  the  chain  relay. 


72 


A  UTOMA  TIC  TELEPHON Y 


Release  is  controlled  by  the  calling  subscriber.  When  he  hangs 
up,  the  line  relay  falls  away,  allowing  the  release  relay  to  de-energize  and 
remove  the  ground  from  the  release  trunk.  Both  line  relay  and  release 
relay  open  the  trunk  circuit.  Thus  the  switches  ahead  and  behind  are 
released. 

The  contacts  of  the  chain  relays  of  all  trunks  in  the  same  group 
are  connected  in  series.  This  chain  or  series  circuit  is  carried  to  a  suitable 
device  which  renders  it  impossible  for  selector  switches  to  attempt  to 
send  calls  through  this  busy  group.  As  long  as  there  is  one  free  trunk  in 
the  group,  this  chain  circuit  is  open  and  calls  may  be  received.  But 
when  all  trunks  are  busy,  the  circuit  is  complete  and  the  stopping  device 
comes  into  play.  A  meter  may  also  be  attached  to  indicate  how  many 
times  this  occurs,  or  a  supervisory  signal  may  be  used. 

The  talking  circuit  of  a  connection  between  offices  is  shown  in  Fig. 
69.  An  important  thing  from  the  standpoint  of  voice  transmission  is 
that  the  current  for  the  transmitter  of  the  calling  telephone  is  supplied 


Repeater 


Connector 


Called 
Telephone 


+  L 


-L 


BBR 


BBR 


Called 
Telephone 


+z 


Fig.   69. — Talking  circuit  of  an  inter-office  connection. 

from  the  originating  office,  and  for  the  called  telephone  from  its  own 
office.     It  results  in  a  high  standard  of  transmission  efficiency. 

Switching  Repeaters. — The  desire  for  economy  of  inter-office  trunks 
between  a  sub-office  and  its  main  office  led  to  the  production  of  the 
switching  repeater  which  was  mentioned  in  the  chapter  on  trunking. 

Strictly  speaking,  there  were  at  one  time  two  kinds,  the  switching 
repeater  and  the  switching-selector  repeater.  The  former  was  a  repeater 
only,  and  for  its  complete  functioning  required  selectors  in  the  sub-office  in 
which  it  was  located.  The  latter  combines  in  itself  the  functions  of 
repeater  and  of  selector,  from  which  it  takes  its  name.  It  is  not  to  be 
confused  with  the  "selector-repeater,"  which  functions  first  as  a  selector 
and  afterwards  as  a  repeater.  The  old  switching  repeater  is  no  longer 
used.  For  the  purpose  of  simplicity  as  well  as  of  clearness,  we  will  use 
the  term  "switching  repeater"  in  describing  the  present  form  of  appa- 
ratus, which  is  primarily  a  repeater  which  acts  as  a  selector  under  part 
of  the  conditions  of  use  and  switches  the  inter-office  trunk  out  of  use. 

At  the  left  of  Fig.  70  is  shown  a  diagram  of  trunking  utilizing  switch- 
ing-selector repeaters.     The  apparatus  inclosed  within  the  dotted  lines 


AUTOMATIC  ELECTRIC  COMPANY'S  APPARATUS 


73 


H04JMS2UI1  RlDLUUd  LUOJJ 


74  AUTOMATIC  TELEPHONY 

represents  the  apparatus  of  a  sub-office,  while  the  remaining  apparatus 
is  assumed  to  be  local  to  the  main  office  At  the  left  of  the  figure  a 
telephone  line  is  shown  terminating  on  a  primary  lineswitch.  These 
lineswitches  have  access  to  trunks  passing  through  a  switching-selector 
repeater  and  terminating  upon  outgoing  secondary  lineswitches.  These 
switches  in  turn  have  access  to  trunks  terminating  upon  incoming  first 
selectors  in  the  main  office.  A  certain  level  in  the  banks  of  the  switching- 
selector  repeater  contains  trunks  which  terminate  upon  connectors  which 
have  access  to  the  lines  in  the  branch  office.  At  the  main  office  a  certain 
level  in  the  banks  of  the  second  selectors  contains  trunks  which  pass 
through  a  repeater  and  terminate  on  primary  lineswitches  at  the  branch 
office.  These  lineswitches  having  access  to  trunks  which  are  a  multiple 
of  those  appearing  in  the  banks  of  the  switching-selector  repeater.  In 
the  figure  the  sub-office  is  represented  as  containing  only  100  lines; 
it  will  readily  be  apparent,  however,  that  the  scheme  is  not  necessarily 
limited  to  an  office  of  this  size. 

In  general,  the  operation  of  the  system  is  as  follows: 
Upon  the  removal  of  the  receiver  from  the  switch-hook  by  the  sub- 
scriber of  one  of  the  sub-office  groups,  his  individual  line-switch  E 
operates  to  select  a  trunk  line  leading  to  a  secondary  switch  H,  which  in 
turn  operates  to  select  an  idle  first  selector  /.  The  subscriber  operates 
his  calling  device  to  cause  the  selected  first  selector  to  extend  connection 
to  an  idle  second  selector  in  the  section  or  division  of  the  exchange  to 
which  the  called  subscriber  belongs.  The  selected  second  selector  is 
then  operated  to  extend  the  connection  to  an  idle  connector  switch  which 
has  access  to  the  group  of  subscribers  to  which  the  called  subscriber 
belongs,  which  connector  is  then  operated  to  establish  connection  with  the 
called  subscriber's  line.  Assuming  now  that  the  called  subscriber  is  in 
the  same  sub-office  as  the  calling  subscriber.  Under  these  conditions, 
when  the  calling  subscriber  operates  his  calling  device  to  cause  the 
second  selector  to  establish  connection  with  a  connector  switch,  the 
switching  apparatus  I,  in  the  trunk  line  which  has  been  seized  by  the 
calling  subscriber's  lineswitch  operates  to  extend  the  connection  directly 
to  one  of  the  connectors  in  the  sub-office.  In  other  words,  when  the 
trunking  has  progressed  to  the  stage  of  eliminating  all  the  sections  except 
the  section  to  which  the  call  is  to  revert,  then  the  switching  apparatus  L 
disconnects  the  call  from  the  trunk  line  leading  to  the  switch  H  and 
switches  the  call  over  to  the  local  connectors  F.  When  the  apparatus  L 
thus  operates  to  extend  the  connection  to  a  local  connector,  it  causes  the 
secondary  switch  //  which  was  previously  selected,  as  well  as  all  the 
selectors  which  have  been  operated  in  the  main  office,  to  be  released  and 
the  connection  is  therefore  confined  solely  to  the  sub-office. 

In    Fig.    70  is    shown   the   circuit  of  a  switching-selector  repeater. 
Numerous  circuits  have  been  developed — the  one  used  here  is  a  repro- 


AUTOMATIC  ELECTRIC  COMPANY'S  APPARATUS  75 

duction  of  the  circuit  appearing  in  the  United  States  Patent  disclosing 
this  type  of  switch  (Patent  1283182  issued  to  Harry  E.  Hershey.) 

This  device  performs  two  functions.  In  a  call  from  the  sub- 
office  to  the  main  office,  it  operates  as  a  repeater  to  repeat  the  impulses 
from  the  calling  sub-station  to  the  trunk  extending  to  the  central  station. 
In  a  local  connection  this  device  operates  to  free  the  trunk  line  leading  to 
the  main  office  and  to  divert  the  call  to  the  proper  local  switches  for 
completing  the  desired  connection.  This  switching  repeater  comprises 
a  vertically-  and  rotably-movable  switch  shaft  which  carries  the  wipers 
P-l,  P-2,  —  W-\-  W  and  is  controlled  by  the  vertical  magnet  V.M.  and  the 
rotary  magnet  R.M.  These  operations  are  controlled  through  the  me- 
dium of  a  double- wound  line  relay  L.R.,  which  is  connected  with  the  line 
conductors  through  the  springs  of  a  reversing  relay  REV.  R.  Y.  The  relay 
is  double-wound  and  is  so  adjusted  that  it  is  operatively  energized  only 
when  both  windings  are  energized  in  the  same  direction.  The  line  relay 
controls  a  second  bridge  across  the  trunk  conductors  for  the  purpose 
of  repeating  impulses  to  the  main  exchange  in  a  manner  to  be  soon 
more  fully  explained.  The  RLSE  MAG.  is  the  usual  release  magnet  for 
restoring  the  switch  shaft  and  side  switch  to  normal  position  upon  the 
release  of  the  connection.  This  release  magnet  is  connected  in  series 
with  a  double-wound  relay  K.  The  switching-selector  repeater  is  also 
provided  with  a  so-called  side  switch  similar  to  that  shown  in  connection 
with  the  test-distributor  circuit. 

There  are  ten  rows  or  levels  of  bank  contacts  for  each  of  the  wipers 
PI,  P2,  —W  and  +W.  The  bank  contacts  accessible  to  the  wipers  P-2, 
—  W  and  +  W  of  the  level  corresponding  to  the  sub-office  in  which  the 
repeater  is  located  are  multiples  between  bank,  while  the  contacts  in 
each  level  of  the  banks  accessible  to  the  wiper  PI  are  connected  to- 
gether, and  the  level  which  corresponds  to  the  group  of  connectors  in 
the  sub-office  is  connected  to  the  spring  of  the  relay  K  of  each  repeater. 
Also  in  the  bank  associated  with  the  wiper  P2  the  first  contact  in  the 
level  corresponding  to  the  main  office  with  which  the  sub-office  is 
connected  with  the  spring  of  the  relay  K.  For  example,  if  the  numbers 
in  the  branch  office  begin  with  the  digit  5,  whenever  the  dial  is  oper- 
ated for  the  first  digit  5,  the  wipers  will  be  raised  to  the  fifth  level,  and 
upon  cutting  in  upon  the  first  contact  in  said  level  will  close  a  circuit 
which  will  cause  the  release  of  this  switch,  at  the  same  time  causing 
a  ground  to  be  placed  upon  all  the  contacts  accessible  to  the  wiper  PI 
which  are  connected  with  the  spring  of  relay  K.  If  the  first  digit  called 
by  the  first  operation  of  the  dial  is  other  than  5,  the  switch  will  not  be 
released  but  will  come  to  rest  upon  the  first  contact  in  the  level  to  which 
it  has  been  raised,  but  should  this  level  be  one  which  leads  to  the  local 
connectors,  the  switching  relay  SW-RY.  will  not  be  operated  for  the  rea- 
son that  at  this  time  there  is  no  ground  on  the  spring  of  relay  K,  as  the 


70  AUTOMATIC  TELEPHONY 

relay  K  has  not  yet  been  energized  by  the  switch  releasing  from  the  fifth 
level.  It  may  be  noted  here  that  should  the  switch  be  resting  upon  a 
bank  contact  leading  to  a  connector,  and  the  switching  relay  SW-RY.  is 
not  energized,  the  contact  in  question  is  not  made  busy,  for  the  switch 
does  not  place  a  ground  upon  its  private  wiper  when  said  wiper  is  in  third 
position.  After  the  first  digit  5  has  been  called,  should  the  next  digit 
called  be  that  of  a  group  of  connectors  at  the  branch  office,  the  switch, 
upon  coming  to  rest  upon  the  first  idle  contact  in  that  level,  will  have 
its  switching  relay  SW-RY.  energized,  due  to  the  ground  now  upon  the 
springs  of  relay  K. 

The  outgoing  secondary  lineswitch  H'  which  corresponds  to  one  of  the 
switches  H,  is  the  same  in  principle  as  the  ordinary  lineswitch,  being 
somewhat  simplified,  however,  in  that  it  is  not  provided  with  line  or  cut- 
off relay  armatures.  This  switch  belongs  to  a  group  controlled  by  a 
master  switch  (not  shown). 

When  the  connection  is  extended  to  the  switching  repeater,  a  cir- 
cuit is  closed  to  the  line  relay.  The  relay,  upon  energizing,  closes  a 
circuit  through  the  relay  R  to  battery.  One  result  of  the  energization 
of  the  relay  R  is  the  closure  of  a  circuit  extending  from  ground  to  the 
release  trunk.  A  further  result  of  this  action  is  the  closure  of  a  circuit 
extending  from  ground  through  the  winding  of  the  secondary  switch  H' 
to  battery.  The  switch  H'  thereupon  operates  to  extend  the  connec- 
tion to  a  first  selector  at  the  main  office.  A  further  result  ©f  the  ener- 
gization of  the  line  relay,  when  the  connection  is  extended  thereto  is 
the  closure  of  a  bridge  across  the  trunk  conductors,  whereby,  as  soon 
as  the  connection  is  extended  to  the  first  selector,  as  above  described, 
an  energizing  circuit  is  closed  for  its  line  relay.  The  relay  REV  RY.  is 
not  energized  at  this  time  because  only  one  winding  has  current  flowing 
through  it. 

The  foregoing  operation  takes  place  immediately  upon  the  removal 
of  the  receiver  from  the  switch-hook  at  the  calling  sub-station.  The 
calling  subscriber  now  operates  his  calling  device  in  the  usual  manner  for 
the  first  digit  5  of  the  called  number.  Each  time  the  sub-station  impulse 
springs  are  separated,  the  line  relay  of  the  switching  repeater  is  momen- 
tarily de-energized,  thus  opening  the  bridge  across  the  trunk  conductors 
thereby  momentarily  breaking  the  energizing  circuit  of  the  line  relay  of 
the  first  selector.  The  relay  R  in  the  switching  repeater  being  slow  acting 
does  not  de-energize  during  the  momentary  interruptions  of  its  circuits 
by  the  relay  LR,  and  therefore  at  each  operation  an  impulse  is  transmitted 
over  a  circuit  extending  from  ground  through  relay  S  vertical  magnet  and 
the  side  switch  wiper  (in  first  position)  to  battery.  The  vertical  magnet 
receives  five  impulses  over  this  circuit  and  operates  to  raise  the  shaft 
wipers  opposite  the  fifth  level  of  bank  contact.  The  slow-acting  relay 
S  is  energized  by  the  first  impulse  which  is  transmitted  through  it  and 


AUTOMATIC  ELECTRIC  COMPANY'S  APPARATUS  77 

remains  in  its  operated  position  until  after  the  last  impulse  is  deliv- 
ered for  the  digit.  In  its  operated  position  the  relay  &  short  circuits 
the  coil  Y  and  the  lower  winding  of  the  relay  REV  RY.,  so  as  to  remove 
their  resistances  from  the  circuit  of  the  line  relay  of  the  selecter  while 
impulses  are  being  transmitted  through  it.  The  operation  of  relay  S 
completes  an  energizing  circuit  for  the  private  magnet  PM.  Upon  the 
breaking  of  this  latter  circuit,  when  the  relay  S  de-energizes  after  the  last 
impulse  for  the  digit  is  delivered  from  the  sub-station,  the  private  mag- 
net de-energizes  and  permits  the  side  switch  to  pass  to  second  position. 
The  side  switch,  upon  passing  to  second  position,  closes  a  circuit  through 
the  rotary  magnet,  which  thereupon  energizes  and  rotates  the  shaft 
wipers  one  step,  upon  the  first  contact  in  the  fifth  level  and  energizes 
the  private  magnet.  The  wiper  P2  will  meet  with  no  guarding  potential 
upon  the  first  contact  of  the  fifth  level,  as  hereinbefore  described,  where- 
upon the  private  magnet  will  deenergize.  At  the  time  that  the  springs 
of  the  private  magnet  are  in  engagement,  a  circuit  may  be  traced  from 
ground  through  the  bank  contact,  upper  winding  of  the  relay  K,  release 
magnet,  to  battery.  The  release  magnet  will  energize  over  this  circuit 
and  operate  to  restore  the  shaft  of  the  switching  repeater  to  normal, 
without  in  any  way  affecting  the  first  selector.  The  relay  K  will  be 
energized  in  series  with  the  release  magnet  and,  upon  operating,  will 
close  a  locking  circuit  for  itself.  The  energization  of  the  relay  K  places 
ground  upon  all  the  bank  contacts  accessible  to  the  wiper  PI  which 
corresponds  to  the  group  of  connectors  in  the  branch  office.  Also, 
places  ground  upon  the  first  contact  in  the  fifth  level  which  is  acces- 
sible by  the  wiper  P2.  The  five  impulses  which  are  transmitted  to 
the  first  selector  by  the  relay  LR  of  the  switching  repeater  caused  five 
impulses  to  be  transmitted  through  the  vertical  magnet  in  the  usual 
manner,  lifting  its  wipers  to  the  fifth  level. 

The  calling  subscriber  now  operates  his  calling  device  for  the  second 
digit  2  of  the  called  number,  in  response  to  which  the  shaft  wipers  of  the 
switching  repeater  come  to  rest  upon  the  first  contact  of  the  second  level 
in  the  bank.  The  second  level  being  dead  in  the  sub-office,  its  first 
contact  will  always  be  idle.  When  the  side  switch  of  the  selector  re- 
peater passes  to  third  position,  a  circuit  is  closed  through  the  lower  wind- 
ing of  the  relay  REV  RY.,  but  the  relay  will  not  pull  up  at  this  time, 
for  the  current  in  its  two  windings  oppose  each  other.  In  response  to 
the  impulses  sent  over  the  trunk  by  the  relay  LR  the  second  selector 
operates  to  place  its  shaft  wipers  upon  the  first  idle  contact  of  the 
second  level  in  its  banks  and  to  extend  the  lines  to  a  connector  in 
the  same  manner  as  explained  for  the  first  selector. 

The  sub-sta.tion  calling  device  is  now  operated  for  the  tens  digit  0, 
whereby  the  switching  repeater  by  means  of  the  line  relay  LR  causes  the 
vertical  movement  of  the  connector  in  the  usual  manner. 


78  AUTOMATIC  TELEPHONY 

The  calling  subscriber  now  operates  his  calling  device  for  the  last 
digit  0,  whereby  the  switching  repeater  by  means  of  the  line  relay  LR 
causes  the  rotary  movement  of  the  connector  in  the  usual  manner. 

Upon  the  response  of  the  called  subscriber  his  line  is  provided  with 
talking  current  through  the  windings  of  the  back-bridge  relay  of  the  con- 
nector, which,  upon  energizing,  reverses  the  flow  of  curent  in  the  trunk 
from  the  switching  repeater.  Since  the  current  in  the  lower  winding  of 
the  relay  REV  RY  is  now  reversed,  the  two  windings  help  each  other  and 
the  relay  becomes  operatively  energized,  and  in  so  doing  reverses  the 
current  in  the  lines  of  the  sub-station. 

After  the  conversation  is  completed,  the  connection  is  released  by  the 
hanging  up  of  the  receiver  at  the  calling  sub-station  in  the  following 
manner:  When  the  receiver  at  the  sub-station  is  restored  to  the  switch- 
hook,  the  separation  of  the  switch-hook  springs  destroys  the  energizing 
circuit  of  the  switching-repeater  line  relay.  The  relay  LR  upon  de-ener- 
gizing, opens  the  circuit  of  the  relay  R  which  in  turn,  upon  de-energizing, 
closes  a  circuit  through  the  release  magnet.  The  release  magnet,  upon 
energizing,  withdraws  the  retaining  pawls  from  the  shaft  and  its  own 
circuit  is  broken  at  the  off-normal  springs  when  the  shaft  reaches  its 
lowest  position.  The  de-energization  of  the  relay  R  also  removes  ground 
from  the  release  trunk  which  allows  the  secondary  switch  H'  to  return  to 
normal  position.  The  de-energization  of  the  relay  LR  opens  the  energiz- 
ing circuit  of  the  line  relay  of  the  connector. 

It  will  now  be  explained  how  a  connection  is  extended  from  the  tele- 
phone station  of  a  sub-office  to  a  telephone  of  the  same  sub-office. 
The  number  of  the  called  telephone  will  be  assumed  to  be  5300. 

Upon  the  removal  of  the  receiver  at  the  sub-station,  the  action  of  the 
lineswitch,  the  switching  repeater,  the  secondary  switch  Hf  and  the  first 
selector  is  the  same  as  previously  described. 

Upon  the  operation  of  the  calling  device  for  the  first  digit  5,  the  switch- 
ing repeater  operates  to  raise  the  shaft  wipers  to  the  fifth  level  and  to 
then  release  therefrom  thus  locking  up  the  relay  K,  as  previously  ex- 
plained.    The  action  of  the  selector  is  the  same  as  previously  explained. 

The  calling  subscriber  now  operates  his  calling  device  for  the  second 
digit  3  of  the  called  number.  As  a  result  the  switching  repeater  raises 
its  shaft  wipers  to  a  position  opposite  the  third  level,  from  which  point 
it  seeks  the  first  idle  contact  in  said  level  of  banks  in  the  following  manner: 
As  soon  as  the  side-switch  wiper  passes  to  second  position,  a  circuit  is 
closed  through  the  rotary  magnet,  which,  upon  energizing,  rotates  the 
shaft  wiper  one  step  and  closes  the  circuit  through  the  private  magnet. 
Should  the  contact  upon  which  the  wiper  P2  is  rotated  have  a  guarding 
potential  upon  it,  a  circuit  will  be  closed  extending  from  ground  at  the 
bank  contact  through  the  private  magnet  to  battery.  In  this  way  the 
private  magnet  will  be  energized  so  long  as  the  wiper  P2  is  passing  over 


AUTOMATIC  ELECTRIC  COMPANY'S  APPARATUS  79 

busy  contacts.  At  the  first  idle  contact,  the  circuit  through  the  private 
magnet  will  be  broken  and  the  private  magnet,  upon  de-energizing,  will 
allow  the  side  switch  to  pass  to  third  position,  thereby  opening  the  circuit 
of  the  rotary  magnet.  The  operation  of  the  first  selector  for  this  digit 
is  the  same  as  previously  explained.  When  the  side  switch  of  the  switch- 
ing repeater  passes  to  third  position,  a  circuit  may  be  traced  from  ground 
through  the  bank  contact  wiper  P-l,  switching  relay  SW  RY  and  the  side 
switch  wiper  in  third  position  to  battery.  The  springs  of  the  relay  SW 
RY  are  adjusted  to  make  contact  with  their  front  contacts  before  they 
break  contact  with  their  back  contact.  The  relay  SW  RY,  upon  becom- 
ing energized  over  the  above  traced  circuit,  operates  to  shift  calling  line 
from  the  line  relay  of  the  switching  selector  to  the  line  relay  of  the  local 
connector.  The  line  relay,  upon  being  disconnected  from  the  line,  de- 
energizes  and  opens  the  circuit  of  the  relay  R,  which  in  turn,  upon  de-ener- 
gizing, allows  the  secondary  switch  H'  to  return  to  normal  position.  At 
the  same  time  the  selectors  in  the  main  office  release. 

Upon  the  subscriber  at  the  sub-station  calling  the  last  two  digits  of 
the  called  number,  the  action  of  the  local  connector  is  the  same  as  usual. 

Upon  the  completion  of  the  conversation,  the  restoring  of  the  receiver 
to  the  switchrhook  at  the  sub-station  causes  the  release  of  the  apparatus 
in  the  usual  manner. 

In  the  foregoing  description  it  has  been  assumed  that  the  switching 
repeater  is  used  in  the  10,000-line  system,  and  that  the  branch  office  is 
not  over  100  lines.  For  this  reason  the  switching  repeater  must  be 
capable  of  operating  upon  the  second  digit  called.  When  used  in  a 
hundred  thousand-line  system,  and  the  branch  office  is  not  above  1000 
lines,  it  is  necessary  for  the  switching  repeater  to  be  capable  of  operating 
upon  the  third  digit  called.  A  slight  modification  of  the  circuit  will 
bring  about  this  result. 

Trunk  Holders. — In  some  cases  two  central  offices  are  so  close  to- 
gether that  it  is  not  necessary  to  have  a  separate  battery  feed.  The  con- 
nector feed  is  ample,  even  to  the  calling  station.  But  it  is  desired  to 
have  only  two-wire  trunks.  In  this  case  it  is  not  worth  while  to  use  the 
repeater,  but  only  a  trunk  holder.     (See  Fig.  71.) 

This  simplest  form  of  trunk  holder  has  only  a  series  relay  LR  to  ener- 
gize a  release  relay  RlRy  to  ground  the  release  trunk  and  to  close  the 
chain  contact.  Both  relays  are  made  slow-release  in  order  to  insure  the 
release  trunk  of  a  ground  during  dialing. 

There  are  conditions  under  which  the  addition  of  parallel  battery 
feed  is  advantageous.  (See  Fig.  72.)  This  is  secured  by  adding  a  differ- 
ential line  relay  DLR,  which  is  in  reality  electrically  polarized.  This 
controls  an  auxiliary  relay  AR  which  connects  the  current  supply  through 
a  double-wound  impedance  coil. 

When  the  trunk  is  seized,  the  line  relay  LR  energizes  the  release  relay 


80 


A  U  TOM  A  TIC  TELEPHON  Y 


RlRy   which    grounds  the  release  trunk  and  polarizes  the  differential 
line  relay  DLR. 

During  the  sending  of  impulses  over  the  line,  the  line  relay  and  the 
release  relay  do  not  vibrate.  The  current  flowing  from  the  distant 
office  over  the  positive  trunk  +  T  is  in  the  wrong  direction  to  actuate  the 
differential  line  relay. 


Rise  Trk. 


LRof 
Incoming 
- 1  Selector 


Fig.   71. — Trunk  holder. 

When  the  called  station  answers,  the  line  current  is  reversed  in  direc- 
tion. This  causes  the  line  winding,  60-ohms,  to  assist  the  polarizing 
winding,  860-ohms,  and  the  differential  line  relay  pulls  up.  This  ener- 
gizes the  auxiliary  relay  AR  which  connects  the  battery  feed  in  parallel 
with  that  of  the  connector  in  the  distant  office  and  with  the  same  polarity. 
The  line  relay  LR  holds  up  with  the  reversed  current.  Though  it  may 
fall  back  momentarily  during  reversal,  the  release  relay  is  so  slow  that 
it  is  not  affected. 

Rise  Trk. 


Fig.   72. — Trunk  holder  with  parallel  battery  feed. 

When  the  subscribers  release,  the  failure  of  current  in  the  line  relay 
LR  permits  it  to  fall  back  and  to  release  the  release  relay.  The  latter 
removes  the  ground  from  the  release  trunk  and  depolarizes  the  differen- 
tial line  relay. 

If  the  calling  subscriber  hangs  up  first,  relay  DR  will  pull  up,  cut-off 
relay  AR  and  the  parallel  battery  feed,  and  hold  RlRy  so  as  to  keep 
the  trunk  busy. 


AUTOMATIC  ELECTRIC  COMPANY'S  APPARATUS 


81 


While  a  trunk  holder  is  useful,  the  inter-office  trunks  are  often  so 
long  and  their  resistance  so  high  that  in  the  interest  of  good  impulsing 
the  regular  form  of  repeater  is  used. 

Quad -Level  Trunks. — Sometimes  the  trunking  between  certain  offices 
is  light.  There  may  not  be  enough  traffic  to  fill  a  ten-trunk  group. 
These  partial  groups  are  not  very  efficient.  Whenever  they  can  be 
combined,  it  should  be  done,  if  it  is  possible  to  separate  the  traffic  at  the 
distant  end  of  the  link  of  trunks. 

Office 


Fig.   73. — Conditions  for  quad-level  trunks. 

Suppose  (Fig.  73)  that  the  total  busy-hour  traffic  from  office  A  to 
the  four  offices  numbered  1,  2,  3,  and  4  is  such  that  from  ten  to  20  trunks 
will  carry  it.  But  because  each  office  is  called  from  a  different  level  on 
the  selectors  at  A,  it  is  ordinarily  impossible  to  combine  them  into  one 
group. 

The  plan  of  quad-level  trunks  (Fig.  74)  permits  this  desired  grouping. 
The  outgoing  trunks  are  multipled  to  four  levels  on  the  banks  of  selectors, 


Infer-offi'ce 


Secffon4 


Trunks 


Quad-level 
I        Repeaters 
Line 
dank 


Switch -selecting         Incoming 
Relays  Selectors 

Fig.  74. — Plan  of  quad-level  trunks. 

but  the  release  trunks  are  separately  attached  to  the  private  bank  con- 
tacts. Each  trunk  passes  through  a  repeater  which  has  four  relays  (A-l, 
A-2,  A-3,  A -4)  affected  by  the  level  selected.  Otherwise  the  repeater 
is  of  the  usual  type. 

At  the  distant  office  where  the  division  takes  place  (Office  1,  in  this 
case)  each  trunk  arrives  at  a  group  of  switch  selecting  relays  D-l,  D-2, 
Z)-3,  and  DA. 

6 


82 


A 11  TOM  AT  IV  TELEPIION  Y 


If  the  subscriber  calls  the  first  level  on  the  selector,  its  wipers  will 
rotate  in  on  the  first  level  and  seize  the  first  idle  trunk.  Relay  A-\  in 
the  repeater  will  be  pulled  up.  This  causes  relay  D-\  in  the  distant 
office  to  pull  up,  switching  the  trunk  to  a  selector  in  section  1,  leading  to 
office  No.  1. 

If  the  subscriber  calls  the  second  level  on  the  selector  in  the  originating 
office,  its  wipers  will  rotate  in  on  the  second  level  and  seize  the  first  idle 
trunk.  Relay  A-2  in  the  repeater  will  be  pulled  up.  This  causes  relay 
D-2  in  the  distant  office  to  pull  up,  switching  the  trunk  to  a  selector  in 
section  2,  which  represents  office  No.  2. 


Switch-selecting  Relays 


"jOTD*  Polar  Relays 


I  Chain  Relays 
Fig.  75. — Quad-level  repeater  and  trunk  circuits. 


The  other  levels  cause  the  direction  of  calls  to  sections  3  and  4  in  a 
similar  manner. 

It  is  not  necessary  to  use  four  groups — three  or  two  can  be  arranged 
just  as  well. 

By  using  the  ten-level  20-trunk  selector,  20  trunks  in  one  group  may 
be  used. 

The  circuits  of  the  quad-level  trunk  system  (Fig.  75)  involve  four 
relays  added  to  the  usual  repeater  and  switch-selecting  relays  in  the 
distant  office. 

The  essential  principle  is  found  in  the  four  polar  relays  in  the  distant 
office  (B-l,  B-2,  B-3,  5-4)  and  the  battery  polarity  put  on  the  two-line 
wires  by  the  relays  A-l,  etc.,  in  the  originating  office. 


AUTOMATIC  ELECTRIC  COMPANY'S  APPARATUS  83 

When  the  subscriber  dials  "one"  the  wipers  of  the  selector  are  lifted 
to  the  first  level.     Then  the  switch  rotates  and  seizes  the  first  idle  trunk. 

The  private  wiper  grounds  the  corresponding  private  contact  on 
the  first  level  of  the  private  bank.  This  pulls  up  relay  A-l,  which  cuts 
off  the  circuits  of  the  other  three  relays  but  not  itself.  It  also  puts 
negative  battery  on  the  negative  trunk  —  T. 

The  negative  battery  on  the  negative  trunk  operates  relay  B-l  in 
the  distant  office,  but  not  B-2,  because  the  latter  is  poled  in  the  opposite 
direction. 

Relay  B-l  energizes  relay  C-l,  which  makes  busy  the  trunk  to  section 
1  and  operates  relay  D-l.  The  latter  switches  the  trunk  lines  to  section 
1.  Relay  C-l  holds  the  ground  on  the  release  trunk  until  the  switch 
ahead  can  pull  up  its  line  and  release  relays.  The  action  of  relays  B-l 
and  C-l  is  momentary,  for  B-l  is  cut  off  by  the  switching  action.  D-l  is 
held  by  the  switch  ahead. 

At  the  same  time  that  the  above  has  been  taking  place,  the  calling 
subscriber  line  has  been  acting  on  the  repeater  proper.  The  line  relay 
pulls  up  and  actuates  the  release  relay  as  usual.  The  latter,  in  addition 
to  other  duties,  grounds  a  wire  leading  to  the  "A"  relays.  This  is  the 
ground  which  holds  relay  ^4-1  and  the  switches  back  of  the  repeater  and 
makes  busy  the  other  private  contacts.  The  release  relay  also  pulls  up 
the  switching  relay.  SwRy,  which  cuts  off  the  selecting  circuit  and  extends 
the  lines  through  to  the  distant  office,  ready  for  dialing  and  talking. 
The  rest  of  the  operations  have  already  been  described  in  connection 
with  Fig.  67. 

Secondary  Lineswitches. — Mention  has  already  been  made  of  second- 
ary lincswitches  in  the  chapter  on  "Trunking. "  These  switches  are 
used  with  economy  in  systems  with  an  ultimate  capacity  of  100,000  lines, 
in  systems  with  an  ultimate  capacity  of  10,000  lines  but  having  more 
than  10  per  cent  trunking,  and  on  trunks  between  comparatively  large 
automatic  offices. 

Both  Keith  type  and  rotary  lineswitches  are  used  as  secondaries, 
the  former  for  local  and  for  out-going  trunks,  the  latter  only  for  the  out- 
going trunks.  A  photograph  of  a  Keith  secondary  lineswitch  is  repro- 
duced in  Fig.  76.     The  rotary  switch  is  like  Fig.  48  except  the  relays. 

As  this  illustration  indicates,  the  mechanical  features  of  the  secondary 
lineswitch  are  the  same  as  those  of  the  primary  lineswitch.  It  uses  banks 
of  the  same  construction  and  is  mounted  on  an  upright  similar  to  those 
used  by  the  primary  lineswitch.  In  fact,  the  only  difference  between  the 
uprights  is  that  lineswitches  are  mounted  on  one  side  only  of  an  upright, 
the  back  of  which  is  reserved  for  connector  switches,  but  switches  are 
mounted  on  both  sides  of  a  secondary  lineswitch  upright,  i.e.,  each  such 
upright,  when  filled,  carries  four  shelves,  each  having  a  capacity  of  50 
lineswitches.     These   lineswitches   may   be   divided  into  four  sections 


SI 


A  UTOMA  TIC  TELEPHONY 


each  employing  a  master  switch,  or  they  may  be  divided  into  eight  sec- 
tions, each  having  a  master  switch.  In  the  latter  event,  one  master 
switch  is  mounted  at  the  customary  place  in  the  middle  of  each  shelf, 
and  the  extra  switch  is  mounted  at  the  top  of  each  shelf.  A  photograph 
of  a  secondary  lineswitches  installed  is  reproduced  in  Fig.  95. 


Fig.   76. — Secondary  lineswiteh,  Keith. 

As  stated  in  the  chapter  on  "Trunking,"  when  the  secondary  line- 
switches  are  inserted  between  the  primary  lineswitches  and  the  first 
selectors,  they  are  used  to  reduce  the  number  of  first  selectors  required, 
by  placing  the  first  selector  switches  in  groups  of  100  instead  of  in  groups 


&witchsts  in 
4-0  qroups  of 
so  switches 

EACH 


2pOO 

IMARY  UlNET 

SWITCHES   IN 

40  GROUPS  OF 

SO  SWITCHES 

EACH 


3?0 

TRUNKS 

IOO 

TRUNKS 

IOO 

TRUWI- 

<S 

320  SKDY. 
LINE     SWS. 

IOO  FIRST 

IOO  SECOND- 
SELECTOR 
SWITCHES 

OF-32  EACH 

320 

^TRUNKS 

IOO 

TRUNI 

<S 

IOO 

TRUNKS 

3eo  secdV. 

LINE   SWS. 
IN  IOO|ROUt=S 
OF32  EACH 

IOO  FIRST 
SELECTOR 
SWITCHES 

'OOSECONO 
SELECTOR 
SWITCHES 

Fig.  77. 


-Diagram  illustrating  scheme  of  using  secondary  lineswitches  to  reduce  first  and 
second  selectors. 


of  ten.  The  application  of  this  mechanism  makes  it  possible  to  give  a 
large  number  of  primary  lineswitches  (say  2000)  access  to  a  group  of  100 
first  selectors. 

The  drawing  in  Fig.  77  shows  the  number  of  primary  lineswitches,  the 
number  of  primary  lineswiteh  groups,  the  number  of  trunks  from  primary 


AUTOMATIC  ELECTRIC  COMPANY'S  APPARATUS 


85 


lineswitches  to  secondary  lineswitches,  the  number  of  secondary  lino- 
switch  sections,  the  number  of  trunks  from  secondary  lineswitches  to  first 
selectors,  and  from  first  to  second  selectors,  that  might  be  used  in  a  typical 
office  of  4000  lines,  using  third  selector  switches. 


Sometimes  secondary  lineswitches  are  used  to  reduce  trunks  between 
main  offices,  and  they  are  then  generally  placed  between  the  first  selector 
switches  and  the  repeaters  on  the  outgoing  trunks.  When  the  secondary 
switches  are  inserted  between  the  lineswitches  and  the  first  selectors,  the 
circuit  is  such  that  when  a  subscriber  lifts  his  receiver  from  the  switch- 
hook,  preparatory  to  making  a  call,  his  primary  lineswitch  and  the  second- 


86  AUTOMATIC  TELEPHONY 

ary  lineswitch  to  which  it  connects  him  operate  almost  in  unison,  so  that 
the  first  selector  secured  is  operated  by  the  first  motion  of  the  dial  as 
usual. 

Figure  78  shows  a  circuit  from  a  calling  telephone  through  a  primary 
and  secondary  lineswitch  to  a  first  selector  trunk. 

The  circuits  of  the  secondary  lineswitch  may  be  divided  into  the 
following  parts:  secondary  lineswitch  proper,  master  switch,  and  relay 
equipment.  The  trunk  from  the  primary  to  the  secondary  lineswitch 
consists  of  four  wires,  the  added  wire  being  called  the  "holding  trunk." 
The  release  trunk  is  arranged  to  be  extended  clear  through  from  the 
primary  lineswitch  to  the  selector  and  later  on  to  other  switches  as  far 
as  it  is  needed.  The  secondary  lineswitch  is  operated  and  held  by  means 
of  the  holding  trunk. 

The  secondary  lineswitch  has  a  pull  down-coil  of  85  ohms  which  is 
powerful  enough  to  pull  in  the  plunger,  and  a  holding  coil  of  1168  ohms 
which,  in  series  with  the  pull-down  coil,  is  able  to  hold  the  plunger  in  the 
bank  with  reduced  current. 

In  order  to  furnish  current  promptly  to  the  BCO  winding  of  the  pri- 
mary switch,  there  is  a  slow  releasing  relay  A  of  10  ohms  resistance  which 
is  normally  in  series  with  the  pull-down  coil.  This  relay  connects  the 
holding  trunk  ground  onto  the  release  trunk  and  holds  it  there  only  until 
the  selector  has  had  time  to  ground  the  release  trunk  directly.  The 
secondary  master-switch  bank  is  grounded  by  means  of  a  contact  in  the 
secondary  lineswitch  bank. 

When  the  subscriber  takes  his  receiver  from  the  hook,  the  primary 
lineswitch  plunges  into  the  bank,  extending  the  line  wires  to  the  trunks, 
-\-T  and  —  T.  The  BCO  is  connected  to  the  release  trunk  and  the  hold- 
ing trunk  is  grounded.  This  ground  simultaneously  actuates  the  primary 
master  switch  and  operates  the  pull-down  coil  of  the  secondary  lineswitch, 
causing  the  plunger  to  be  driven  into  the  bank.  At  the  same  time  the 
10-ohm  relay  A  receives  current  enough  to  pull  up  and  ground  the  release 
trunk,  so  that  the  BCO  of  the  primary  lineswitch  will  hold  the  plunger 
in  the  bank,  although  its  pull-down  coil  is  cut  off. 

The  action  of  the  secondary  lineswitch  cuts  out  the  relay  A  and  sub- 
stitutes in  its  place  the  holding  coil  of  1168  ohms,  by  means  of  which  the 
plunger  is  retained.  The  secondary  lineswitch  extends  the  trunks  -\-T 
and  —  T  and  release  trunk  to  the  first  selector.  It  also  grounds  the 
secondary  master  bank  so  as  to  cause  the  idle  plungers  to  be  moved  to 
another  trunk. 

The  selector  line  relay  at  once  pulls  up  and  operates  the  release  relay, 
placing  a  ground  upon  the  release  trunk  so  that  when  the  relay  A  falls 
back  the  connection  will  still  be  retained. 

During  a  conversation,  both  lines  are  clear  from  attachments  all  the 
way  from  the  telephone  to  the  switches  beyond  the  lineswitches.     The 


AUTOMATIC  ELECTRIC  COMPANY'S  APPARATUS  87 

release  trunk  runs  through  from  BCO  of  primary  switch  without  attach- 
ments. The  holding  trunk  is  local  to  the  primary  and  secondary 
switches. 

Chain  Relays. — The  purpose  of  the  chain  relays  and  their  accompany- 
ing circuits,  which  are  installed  where  secondary  lines  witches  are  used,  is 
to  prevent  calls  being  lost  when  all  of  the  outgoing  trunks  from  any  group 
of  secondary  lineswitches  are  busy.  When  a  primary  lineswitch  releases 
its  plunger  arm,  after  having  occupied  a  trunk  to  a  sedondary  lineswitch 
group,  the  plunger  does  not  engage  the  master-switch  shaft  at  once 
but  remains  out  of  engagement  with  the  shaft  and  still  poised  over  the 
trunk  recently  used  by  it,  until  the  master  shaft  in  the  regular  course  of 
events  "picks  it  up,"  i.e.,  moves  into  a  position  where  the  plunger  slips 
into  engagement. 

Meanwhile,  if  the  subscriber  to  whose  line  the  plunger  under  discussion 
belongs  should  lift  his  receiver  from  the  switch-hook,  the  plunger  will 
again  connect  him  to  the  trunk  used  in  his  last  call.  It  therefore  some- 
times happens  that  a  busy  subscriber,  who  is  making  a  number  of  calls  in 
succession,  will  use  the  same  trunk  repeatedly.  No  difficulty  could 
possibly  arise  from  this  unless  secondary  lineswitches  are  used.  Where 
secondaries  are  used,  the  trunks  to  each  secondary  lineswitch  unit  do  not 
come  from  any  one  primary  lineswitch  group  but  are  distributed  as 
widely  and  as  evenly  as  possible  among  the  lineswitch  groups  or  shelves. 

The  loss  of  a  call  may  occur  as  follows: 

Suppose  that  a  subscriber's  line  which  has  been  extended  from  his 
primary  lineswitch  through  secondary  section  No.  3  to  a  selector  releases, 
and  the  primary  master  switch  does  not  immediately  pick  up  his  plunger. 
Suppose  that  secondary  section  No.  3  now  becomes  fully  occupied  by  calls 
from  other  primary  sections.  If  now  our  subscriber  calls  again,  his 
primary  lineswitch  will  have  but  one  outlet,  viz.,  to  the  busy  secondary 
section  No.  3,  into  which  it  can  not  connect  because  the  "open-main" 
wire  is  open.  There  is  no  danger  of  double  connection.  Our  subscriber 
will  lose  his  call. 

The  chain  relays  prevent  this  condition  by  closing  a  circuit  whenever 
all  of  the  trunks  outgoing  from  any  secondary  lineswitch  group  are  busy, 
which  causes  the  master  switches  of  all  of  the  primary  lineswitch  groups 
having  trunks  to  the  busy  secondary  group  to  sweep  their  shafts  through 
the  entire  arc  and  thereby  pick  up  any  plunger  which  may  be  released 
but  out  of  engagement  with  the  master-switch  shaft.  For  this  purpose  a 
number  of  relays  co-operate. 

When  the  last  one  of  ten  trunks  becomes  busy,  its  chain  relay  simul- 
taneously grounds  the  only  remaining  master-switch  bank  contact  and 
closes  the  chain  circuit  of  the  stop  relay.  The  latter  prevents  the  master 
switch  from  vainly  hunting  for  the  idle  trunk  which  does  not  exist.  It 
also  grounds  the  "kick-off"  wire,  which  ground  is  very  soon  removed  by 


88  AUTOMATIC  TELEPHONY 

the  falling  back  of  the  D  relay.  The  latter  was  cut  off  by  the  start  re- 
lay, because  the  latter  pulled  up  at  the  same  time  as  the  stop  relay. 

The  momentary  pulsation  over  the  kick-off  wire  operates  all  the  kick- 
off  relays  D  in  the  master  switches  whose  trunks  run  to  the  secondary 
section  which  now  is  busy.  Each  relay  D  locks  itself  through  the  back 
contact  of  the  trip  relay  of  the  primary  master  switch.  It  at  the  same 
time  closes  contacts  in  parallel  with  the  start  relay,  so  as  to  cause  the 
master  switch  to  let  its  guide  shaft  rotate  to  trunk  No.  1.  Here  the  trip 
relay  is  brought  into  play  and  locked  as  was  described  before.  The 
solenoid  pulls  the  shaft  to  trunk  No.  10,  on  the  way  picking  up  all  the 
idle  plungers.  The  operation  of  the  trip  relay  unlocked  the  relay  D, 
so  that  on  arriving  at  trunk  10  the  master  switch  will  be  free  to  choose 
trunks  as  before. 

During  the  time  that  the  secondary  section  is  busy,  the  open  main  wire 
is  grounded  by  relay  D.  The  total  resistance  from  primary  master  switch 
bank  contact  through  secondary  lineswitch  to  ground  is  not  much  over 
95  ohms,  consisting  of  relay  A  and  PDC.  This  prevents  any  primary 
master  switch  from  stopping  on  this  trunk. 

The  circuit  of  the  secondary  master  switch  has  been  rearranged  for 
compactness  and  clearness  of  current-flow.  The  mechanical  relations 
and  functions  of  the  various  parts  are  like  those  of  the  primary  master 
switch.  "F-l"  is  the  finger  which  operates  at  trunk  1  while  "F-10"  is 
the  finger  which  operates  at  trunk  10.  The  supervisory  relay  for  the 
master  switch  has  to  be  in  the  negative  lead,  because  the  mainspring  of 
the  start  relay  furnishes  ground  normally  to  relay  D.  The  latter  is  nor- 
mally energized,  and  the  current  drawn  by  it  would  interfere  with  the 
proper  operation  of  the  supervisory  relay. 

During  the  time  that  the  secondary  master  switch  is  hunting  for  an 
idle  trunk,  none  of  its  lineswitches  must  be  allowed  to  plunge  into  the 
bank.  This  is  taken  care  of  by  the  open  main.  But  if  the  hunting  time 
should  be  a  little  too  long,  one  or  more  calls  might  be  stored  up,  so  that 
when  the  idle  trunk  is  found  and  battery  connection  restored  to  the 
open  main,  more  than  one  secondary  lineswitch  might  be  plunged  in  on 
the  same  trunk  and  a  double  connection  result. 

Double  connection  in  the  above  case  is  prevented  by  relay  D.  During 
brief  operations  of  the  master  switch  it  will  remain  energized.  But  if 
the  hunting  period  be  longer  than  is  safe,  relay  D  falls  back,  grounds  the 
holding  trunk,  and  prevents  any  primary  master  switch  from  sending 
calls  over  this  trunk. 

The  choke  relay  performs  a  function  which  is  not  obvious.  It  pre- 
vents the  primary  master  switch  from  running  past  a  single  idle  trunk, 
which  easily  occurs  if  the  holding  trunk  and  open  main  of  secondary  are 
connected  to  negative  battery  through  a  low  resistance.  While  the 
primary  master  switch  is  sweeping  over  its  bank,  its  wiper  may  pass  over 


AUTOMATIC  ELECTRIC  COMPANY'S  APPARATUS 


89 


a  succession  of  grounded  contacts,  followed  by  a  single  one  which  is  not 
grounded.  The  start  relay  is  energized  When  it  arrives  at  the  single 
ungrounded  contact,  the  start  relay  is  supposed  to  fall  back  promptly 
and  stop  the  guide  shaft.  But  if  the  master  bank  has  on  the  idle  con- 
tact a  low  resistance  path  to  negative  battery,  the  latter  acts  like  a  shunt 
on  the  start  relay,  making  it  temporarily  slow  to  release.  It  will  not  fall 
back  quickly  enough,  thus  permitting  the  shaft  to  pass  on  beyond. 

The  choke  relay  has  normally  a  high  impedance,  so  that  under  the 
above  conditions,  it  does  not  affect  the  start  relay  of  the  primary  switch. 
But  when  the  primary  switch  places  a  ground  on  the  holding  trunk, 


Ou tgoing  5econday  L  meswi  ten  j 


Parfof 
Repeater 


\ 


RkeJ 


85M 

^  Hold       ,„ 
*t  Co''     ,lhf 


•H'I'I"- 


ijir1"      Cham  Relay 

I  Xym&fbrthisTrunk 

1  H"l'r+ 


Fig.  79. — Circuit  of  outgoing  secondary  lineswitch,  Keith  type. 

the  choke  relay  pulls  up  and  short-circuits  all  of  its  w'nding  except  18 
ohms,  becoming  little  more  than  a  conductor. 

Outgoing  Trunk  Secondary  Lineswitches. — Both  types  of  line- 
switches,  Keith  and  rotary,  have  been  used  on  outgoing  trunks  from  one 
office  to  another.  They  are  located  between  the  banks  of  the  selectors 
and  the  repeaters. 

Figure  79  shows  the  circuits  of  a  Keith  outgoing  secondary  lineswitch 
with  its  master  switch  and  associated  relays.  The  circuits  are  much 
simpler  than  those  for  the  local  secondaries,  because  no  pick-up  device 
is  needed  and  the  selectors  now  used  are  not  affected  by  low  resistance 


90 


AUTOMATIC  TELEPHONY 


connections  to  negative  battery.  There  is  also  no  danger  of  premature 
release,  because  the  release  trunk  is  grounded  amply  long  by  the  selector. 

The  lineswitch  proper  has  only  the  pull-down  coil,  PDC,  and  the  hold- 
ing coil.  The  chain  relays  perform  the  same  functions  as  before.  The 
stop  relay  of  the  master  switch  needs  only  to  stop  the  master  switch  by 
cutting  off  the  battery,  for  the  start  relay  switches  the  open  main  from 
negative  battery  to  ground  to  busy  the  trunks  while  hunting  for  an  idle 
trunk. 

The  skeleton  of  the  circuits  arriving  from  the  left  through  the  selector 
shows  the  selector  in  the  act  of  seizing  the  trunk.  The  ground  on  the 
release  trunk  has  not  yet  been  removed.     Here  we  begin. 

The  ground  on  the  release  trunk  in  the  selector  operates  the  pull-down 
coil  PDC  and  drives  the  plunger  into  the  bank.  This  extends  the  lines 
and  release  trunk  clear  through  to  the  repeater,  which,  by  its  line  relay 


— — lk6^>v 


J  ^c. 


PW 


\  rTTTii  Cham  Re/at/ 


JChainRelays 
ofOtherTwnks 
in  Same  Section 


Fig.   80. — Circuit  of  outgoing  rotary  secondary  lineswitch. 

and  release  relay,  places  a  ground  on  the  release  trunk.  The  selector 
may  now  remove  its  ground.  The  lineswitch  springs  switch  in  the  hold- 
ing coil  to  hold  the  plunger  on  reduced  current. 

The  lineswitch  bank  grounds  the  master  switch,  so  that  it  moves  its 
idle    plungers  to  an  idle  trunk  in  the  usual  manner. 

During  conversation  the  lines  are  clear,  and  the  release  trunk  has 
attached  to  it  only  the  holding  coil  of  the  lineswitch.  Release  is  accom- 
plished by  merely  removing  the  ground  from  the  release  trunk  and  letting 
the  plunger  withdraw  from  the  bank. 

If  the  mechanism  of  the  master  switch  stops  between  positions,  the 
lock  magnet  will  be  unable  to  return  to  normal.  This  leaves  the  lock- 
magnet  contact  closed,  and  the  1000-ohm  winding  of  the  supervisory 
relay  therefore  energized.  In  addition  to  lighting  the  supervisory  lamp, 
it  switches  the  open  main  from  negative  battery  to  ground,  so  that  as 
long  as  the  master  switch  is  out  of  order  no  calls  will  be  received.  The 
trunk  will  test  busy. 


AUTOMATIC  ELECTRIC  COMPANY'S  APPARATUS  91 

The  rotary  outgoing  secondary  lineswitch  (Fig.  80)  has  circuits  which 
are  still  more  simple.  Each  switch  has  a  motor  magnet  MM,  line  relay 
LR,  and  a  cut-off  relay  COR.  But  the  only  common  apparatus  is  the 
chain  relays  and  the  group  busying  relay. 

When  a  selector  seizes  this  trunk,  it  extends  the  lines  so  that  the 
telephone  draws  current  from  the  line  relay  and  battery  connections  in 
the  rotary  lineswitch  as  shown.  The  line  relay  pulls  up  and  grounds  the 
release  trunk,  so  that  the  selector  ground  may  be  taken  off.  It  also 
connects  the  magnet  MM  to  the  private  wiper  PW  through  contacts 
1-2  of  the  cut-off  relay.  This  is  to  test  the  trunk.  Contact  Y  closes  a 
little  later  than  contact  X,  grounding  the  cut-off  relay  winding.  Since 
the  cut-off  relay  and  the  magnet  are  in  series,  it  is  essential  that  the  con- 
tact X  shall  close  first,  so  that  if  the  trunk  is  busy  (grounded)  the  cut-off 
relay  will  not  pull  up,  but  will  give  the  magnet  time  to  rotate  the  wipers 
to  the  next  trunk. 

If  the  trunk  is  idle,  the  magnet  will  find  no  ground.  This  leaves  the 
cut-off  relay  free  to  energize.  It  switches  the  lines  from  line  relay  and 
ground  to  the  line  wipers  +  W  and  —  W,  cuts  the  release  trunk  through 
to  the  private  wiper  PW,  and  connects  the  winding  of  the  relay  to  the 
release  trunk  so  as  to  hold  the  cut-off  relay  energized  during  the  holding 
time.  The  repeater  immediately  places  a  ground  on  the  release  trunk, 
so  that  the  line  relay  of  the  rotary  lineswitch  may  fall  back  without  harm. 

If  the  trunk  is  busy  and  the  private  wiper  therefore  grounded,  the 
action  of  the  line  relay  will  result  in  there  being  a  ground  at  both  ends  of 
the  cut-off  relay  winding,  so  that  it  can  not  act.  The  private  wiper 
ground  permits  the  magnet  MM  to  act,  driving  the  wipers  to  the  next 
trunk.  Since  the  magnet  drives  the  wipers  on  the  back  stroke,  the 
magnet  is  de-energized  when  the  wipers  strike  the  next  set  of  contacts. 
If  they  also  are  grounded,  the  cut-off  relay  is  again  short-circuited,  and 
the  magnet  performs  again.  If  they  are  not  grounded,  the  magnet  can 
not  pull  up,  but  the  cut-off  relay  will  act  as  before  described. 

During  conversation  the  lines  are  free,  and  the  release  trunk  has  only 
the  cut-off  relay  attached.  At  release,  the  cut-off  relay  merely  falls  back, 
the  wipers  remaining  where  they  are. 

Party  Line  Equipment. — Selective  ringing  party-line  equipment  is  fur- 
nished by  the  Automatic  Electric  Company  for  use  in  connection  with  its 
local  battery  three-wire  system,  its  common  battery  three- wire  system 
and  its  two- wire  system;  but,  since  the  general  method  of  operation  is 
quite  similar  in  all  of  these  systems,  a  detailed  explanation  will  be  given 
of  the  circuits  used  with  the  two-wire  system  only. 

Two,  three,  four  or  five-party-line  service  is  supplied.  The  selective 
ringing  feature  of  the  telephone  generally  consists  of  what  is  called  a 
"harmonic  ringer;"  that  is  to  say,  a  ringer  whose  clapper  is  mounted  on  a 
tuned  reed  instead  of  on  pivots. 


92  AUTOMATIC  TELEPHONY 

Two  systems  of  frequencies  have  come  into  use,  the  first  of  which  had 
its  origin  in  the  limitations  of  motor-generators.  The  multiple  harmonic, 
system  employs  the  frequencies  16.7,  25,  33.3,  50.0,  and  66.7  cycles  per 
second..  The  non-multiple  harmonic  system  employs  the  frequencies 
20,  30,  42,  54,  and  66  cycles  per  second.  There  is  nothing  about  the 
construction  of  these  ringers  which  is  peculiar  to  automatic  systems. 
Ringers  of  similar  design  are  furnished  by  manufacturers  of  manual 
telephones  and  switchboards.  Two-party  line  service  is  sometimes  given 
by  using  ordinary  ringers  and  connecting  a  ringer  of  one  telephone 
between  the  positive  side  of  the  line  and  earth,  and  the  ringer  of  the  other 
telephone  between  the  negative  side  of  line  and  earth.  This  also  is 
commonly  used  in  manual  telephone  practice. 

Central  Office  Equipment. — Two  arrangements  for  ringing  harmonic 
party  lines  are  in  use.  The  first  employs  a  group  of  connectors  for  each 
frequency.  The  second  uses  connectors,  each  of  which  is  equipped  with 
a  frequency  selector.  The  latter  is  operated  by  one  of  the  latter  digits  of 
the  call  number. 

Groups  of  Connectors. — On  a  five-party  line  each  telephone  has  its 
own  individual  number.  These  numbers  are  generally  assigned  so  that  to 
100  five-party  lines  a  consecutive  series  of  500  numbers  will  be  assigned; 
for  example,  a  series  like  4100,  4200,  4300,  4400  and  4500.  With  this 
particular  series  the  numbers  given  to  the  individual  telephones  on  line  24 
of  this  one  hundred  lines  would  be  "4124,"  "4224,"  "4324,"  "4424"  and 
"4524;"  while  the  numbers  assigned  to  the  telephones  on  line  36  would  be 
"4136,"  "4236,"  4336,"  4436"  and  "4536." 

A  set  of  connector  switches  is  installed  for  each  of  the  hundreds  of 
numbers  in  the  usual  way;  one  set  for  the  "4100"  numbers,  a  second  set 
for  "4200,"  a  third  for  "4300,"  a  fourth  for  "4400"  and  a  fifth  for  "4500." 
The  banks  of  these  five  sets  of  connector  switches  are  multipled  together 
so  that  if  a  subscriber  has  called  number  "4224,"  for  example,  and  has 
consequently  placed  a  guarding  potential  on  the  private  bank  contact 
corresponding  to  that  number,  this  guarding  potential  will  be  established 
through  the  multiple  on  the  private  bank  contacts  of  "4124,"  "4324," 
"4424"  and  "4500"  also.  Therefore,  only  one  of  the  five  parties  on  a  line 
may  be  called  at  a  time,  and  any  one  attempting  to  call  that  party, 
or  any  of  the  others  on  the  line,  while  that  party  is  using  it,  will  receive 
the  customary  busy  signal. 

Selective  ringing  is  easily  and  simply  accomplished,  by  supplying  16.7 
cycle  ringing  current  to  the  ringing  relay  busbars  of  the  "4100"  group  of 
connector  switches,  25-cycle  ringing  current  to  the  ringing  busbars  of  the 
"4200"  group  of  connector  switches,  33.3-cycle  ringing  current  to  the 
busbars  of  the  "4300"  group,  50-cycle  ringing  current  to  the  busbars 
of  the  "4400"  group  and  66.7-cycle  current  to  the  "4500"  gruop.  Con- 
sequently, when  a  subscriber  calls  "4424"  he  secures  the  same  line  that 


AUTOMATIC  ELECTRIC  COMPANY'S  APPARATUS  93 

he  would  secure  if  he  called  "4124,"  but  projects  50-cycle  ringing  cur- 
rent on  to  the  line  instead  of  16.7-cycle,  and  the  only  bell  which  is  rung 
is  that  of  telephone  "4424." 

While,  as  already  stated,  there  is  a  separate  group  of  connector 
switches  for  each  100,  these  groups  are  smaller  than  those  installed  for 
calling  straight  line  numbers,  and,  as  explained  in  the  chapter  on  traffic, 
for  five-party  line  service  it  is  customary  to  install  five  connector  switches 
in  each  group.  For  two-party  line  service,  seven  connector  switches  are 
generally  put  in  each  group. 

Although  five  numbers  are  used,  only  one  lineswitch  is  necessary  for 
each  line;  therefore,  100  party  lines  are  served  by  one  lineswitch  unit, 
which  carries  on  one  side  the  customary  set  of  100  lines  witches  with  their 
master  switches,  and  on  its  other  side,  the  five  groups  of  connector 
switches,  to  give  five-party  line  service;  or,  the  two  or  three  groups  of 
connector  switches  required  to  give  two  or  three-party  line  service. 

The  banks  of  each  group  of  connector  switches  are  brought  out  to 
terminals  and  are  there  connected  together  by  jumpers.  This  is  done  so 
that  if  any  number  on  a  party  line  should  be  out  of  use,  but  still  appear  in 
the  directory,  and,  therefore,  be  subject  to  calls  by  subscribers,  the  bank 
multiple  corresponding  to  this  particular  number  can  readily  be  dis- 
connected at  the  terminal  from  the  multiple  of  the  remaining  numbers, 
and  be  connected  through  a  dead-number  trunk  to  the  information 
operator's  desk. 

When  one  subscriber  on  a  party  line  has  secured  a  connection,  none  of 
the  other  subscribers  can  release  him,  because  a  connection  can  only  be 
released  by  opening  the  circuit,  which  does  not  occur  until  all  of  the  sub- 
scribers on  the  line  hang  up  their  receivers. 

Connectors  with  Frequency  Selectors. — The  circuit  of  a  connector 
equipped  with  a  frequency  selector  (Fig.  81)  is  the  same  as  that  of  the 
connector  shown  in  Fig.  60,  with  the  addition  of  the  minor  switch  with 
five  bank  contacts.  It  is  arranged  so  that  the  hundreds  digit  operates 
the  frequency  selector,  after  which  the  tens  and  units  digits  cause  the 
lifting  and  rotating  of  the  shaft  as  usual. 

The  minor  switch  (Fig.  82)  was  developed  for  general  utility  work 
and  may  have  various  numbers  of  bank  contacts.  It  has  its  own  release 
magnet.  As  used  in  this  connector  circuit,  one  wiper  is  normally  on  the 
first  contact  and  acts  as  an  off-normal  switch.  The  other  wiper  stands 
off  the  bank  and  makes  connection  with  the  first  contact  when  the  magnet 
FSM  makes  one  step. 

The  operation  (Fig.  81)  is  as  follows:  When  the  connector  is  seized, 
the  line  relay  and  release  relay  pull  up.  The  latter  grounds  the  release 
trunk,  closes  the  path  from  the  back  contact  of  the  line  relay  to  the  off- 
normal  springs  ONS,  and  energizes  the  ring  cut-off  relay  RiCOR  through 
the  300-ohm  resistance  and  the  200-ohm  winding  of  the  relay.     Thus  a 


94 


A  UTOMA  TIC   TELEPHONY 


circuit  is  prepared  from  the  back  contact  of  the  line  relay,  through 
the  front  contact  of  the  release  relay,  off-normal  springs  ONS,  front 
contact  of  ring  cut-off  relay,  back  contact  of  wiper-closing  relay  WCRy  to 
the  frequency  selector  magnet  FSM  and  negative  battery. 


Rise  Trk 


Busy 
Tone  - 


-m^y 


RhofCon) 


,—P^HllllH- 
4*  t-+ 


y 


RotRy\ 


4Hi|iIh 
MB/) 


-umiiiui 


125"  1300^1 


....[ 


'ff*' 


54321  <;fe,^~ 

-j:       Thru  Interrupter  orarr 

VJI     fo  Ringing  Oenervrtvrs 

Fig.   81. — -Connector  with  frequency  selector. 


*    s 


Fi<;.   S:*.-     Minor  switch. 


PVV 


-W 


When  the  subscriber  pulls  the  hundreds  digit,  the  pulsations  formed 
by  its  back  contact  operate  the  frequency-selector  magnet  FSM,  rotating 
the  wipers  of  the  minor  switch.  Pulsations  formed  by  the  contact  of 
this  magnet  keep  the  ring  cut-off  relay  energized.     The  latter  depends 


AUTOMATIC  ELECTRIC  COMPANY'S  APPARATUS  95 

solely  upon  the  magnet,  because  the  first  step  of  the  minor  switch  breaks 
the  connection  through  the  300-ohm  resistance. 

At  the  end  of  the  series  of  impulses,  the  ringing  wiper  of  the  frequency 
selector  stops  on  the  contact  desired,  and  the  ring  cut-off  relay  falls 
back.  This  latter  action  switches  the  impulse  circuit  of  the  line  relay 
onto  the  vertical  magnet  VM  by  way  of  the  winding  of  the  series  relay 
Se.  The  action  from  here  has  been  described  in  connection  with  Fig. 
60,  and  need  not  be  repeated. 

The  release  of  Fig.  81  is  in  general  the  same  as  that  of  the  connector 
of  Fig.  60,  except  for  the  addition  of  the  minor  switch.  The  release 
magnet  of  the  latter  is  wired  in  parallel  with  the  release  magnet  of  the 
major  switch.  When  the  line  relay  and  the  back-bridge  relay  fall  back, 
they  and  the  release  relay  close  the  path  for  both  release  magnets  named. 
This  circuit  is  opened  when  the  shaft  has  dropped  to  normal,  thus  return- 
ing the  off-normal  springs  to  normal. 

If  the  subscriber  hangs  up  after  he  has  seized  the  connector,  but 
before  he  has  dialed  any  number  on  it,  the  release  magnet  circuit  is 
completed  through  the  springs  of  the  ring  cut-off  relay  instead  of  the 
off-normal  springs.  The  reason  for  this  is  that  in  releasing,  the  first  act 
of  the  line  relay  is  to  operate  whatever  magnet  is  at  that  time  connected 
to  its  back  contact.  Initially  it  is  the  magnet  of  the  frequency  selector, 
FSM.  This  magnet,  together  with  the  ring  cut-off  relay  will  remain 
energized  until  the  release  relay  falls  back.  Then  the  circuit  of  the 
release  magnets  wil  close  and  will  not  open  until  the  ring  cut-off  relay 
falls  back,  which  will  be  after  the  usual  delay  of  a  slow-release  relay. 
The  time  is  ample  for  the  frequency  selector  to  return  to  normal.  The 
momentary  operation  of  the  release  magnet  of  the  connector  does  no 
harm. 

Reverting  Calls. — This  term  is  applied  to  a  call  of  one  subscriber  on  a 
party  line  for  another  subscriber  on  the  same  line.  It  is  entirely  feasible 
to  arrange  it  so  that  a  calling  subscriber  need  not  know  when  he  is  calling 
another  on  his  own  line,  and  will,  therefore,  make  the  call  in  the  same  way 
that  he  would  if  he  was  calling  a  party  belonging  to  an  entirely  different 
line.  While  this  is  a  desirable  feature,  the  number  of  reverting  calls  is 
usually  small,  and  it  s,  therefore,  not  considered  good  practice  to  put  in 
the  special  lineswitches  and  connector  switches  required  for  handling 
them  in  this  manner. 

The  customary  method  is  to  provide  in  the  central  office  a  number  of 
"reverting-call  switches"  which  are  accessible  to  first  selectors  or  second 
selectors.  Each  party  line  telephone  is  provided  with  an  instruction 
card  giving  the  numbers  of  the  other  stations  on  the  same  line  with  a 
special  number  opposite  each.  The  subscriber  is  instructed  to  call  the 
special  number,  hang  up  his  receiver  and  wait  while  the  desired  station  is 
being  rung. 


96 


AUTOMATIC  TELEPHONY 


The  reverting-call  switch  rings  alternately  the  called  station  and  the 
calling  station.  When  the  called  party  answers,  the  ringing  stops. 
Then  the  calling  subscriber  takes  down  his  receiver  and  the  conversation 
proceeds. 

The  act  of  the  called  subscriber  in  answering  releases  the  reverting- 
call  switch  and  the  selectors  through  which  it  was  reached.  But  imme- 
diately a  first  selector  is  taken  and  it  supplies  talking  battery  current 
to  the  two  telephones.  When  both  of  them  hang  up  their  receivers,  the 
first  selector  and  the  lineswitch  return  to  normal.  Thus  the  reverting- 
call  switch  is  used  only  long  enough  to  ring  the  bells — it  is  not  held 
during  conversation. 

The  circuit  of  a  reverting-call  switch  is  given  in  Fig.  83.  Its  chief 
features  are  the  usual  vertical  and  rotary  magnets  and  the  release  magnet, 


/  Private  Bank 


RIse.Trk. 


Rises®.  'j^J^n 

<-***.  0.  N.5 


SAMPL  E  BANK  COMBINATIONS: 

d:~js  l^and  33^ Bells  Across  the  Line  or  Both  from-Lto  Earth 
75  IG'and 50^  dells  Across  the  Line  or  Both  from-L  to  Earth 


Any  Other  j'^fj 

tlau  Be  Hade]  "3" Rings  50^  Bell  on  -  L  and50~Bell  e>n+L(Earth) 
*  { "4" Rings,  SO^Bel/on  -L  and  66^  dell  on+L  (Earth) 


Fig.  83. — Reverting  call  switch  circuit. 

controlled  by  the  line  relay  and  the  release  relay;  the  relays  for  directing 
the  ringing  current  to  either  side  of  the  line  and  for  cutting  it  off,  and  the 
wipers  and  banks  arranged  to  supply  the  ringing  current  in  all  the 
desired  frequencies  and  for  controlling  the  directing  relays. 

When  the  calling  subscriber  has  dialed  the  first  part  of  the  special 
number,  he  is  connected  to  the  incoming  lines  of  the  reverting-call 
switch.  (See  Fig.  83.)  The  line  relay  LR  immediately  pulls  up,  followed 
as  usual  by  the  release  relay  RlRy.  The  latter  prepares  part  of  the  im- 
pulsing circuit,  prepares  a  ground  for  the  starting  relay  StRy,  and  grounds 
the  release  trunk.  The  latter,  in  addition  to  holding  the  connection 
through  other  switches,  pulls  up  the  directing  relay  DrRy.  The  directing 
relay  switches  the  impulsing  circuit  to  the  vertical  magnet,  and  prepares 
a  ground  to  lock  itself. 


AUTOMATIC  ELECTRIC  COMPANY'S  APPARATUS  97 

The  first  series  of  impulses  sent  to  the  switch  operates  the  vertical 
magnet,  lifting  the  wiper  shaft  to  the  desired  level.  During  the  series, 
the  private  cut-off  relay  PCOR  is  energized,  being  in  parallel  with  the 
vertical  magnet.  The  private  cut-off  relay  closes  the  locking  circuit 
of  the  directing  relay,  so  that  although  the  off-normal  springs  ONS 
open,  the  directing  relay  will  be  held  energized  to  the  end  of  the  series. 

When  the  first  series  of  impulses  end,  the  private  cut-off  relay  falls 
back,  permitting  the  directing  relay  to  release.  This  switches  the  im- 
pulsing circuit  of  the  line  relay  to  be  directed  to  the  rotary  magnet. 

The  second  series  of  impulses  causes  the  rotary  magnet  to  rotate  the 
wiper  shaft  to  the  desired  set  of  contacts.  At  the  same  time  the  private 
cut-off  relay  pulls  up  and  remains  energized.  It  cuts  off  the  two  private 
wipers,  P-l  and  P-2,  and  also  cuts  off  the  circuit  of  the  starting  relay 
StRy. 

When  the  second  series  of  impulses  ends,  the  private  cut-off  relay 
falls  back,  connecting  three  relays  so  that  they  may  act  if  the  conditions 
favor  their  action.  The  directing  relay  is  connected  to  P-l,  the  fre- 
quency relay  FR  to  P-2,  and  the  starting  relay  to  the  line  which  is  nor- 
mally connected  to  the  line  wipers. 

The  connections  on  the  private  bank  and  on  the  line  bank  determine 
what  frequencies  shall  be  alternately  rung,  and  how  they  shall  be  applied 
to  the  subscriber  line.  A  few  sample  connections  are  shown  to  illus- 
trate the  plan.  Many  other  connections  are  needed  to  ring  all  pairs  of 
stations. 

In  general,  P-l  and  the  directing  relay  cause  the  ringing  current  to 
be  directed  to  one  line  wire  or  to  the  other,  while  P-2  and  the  frequency 
relay  take  care  of  the  frequency  or  frequencies  to  be  sent  out. 

If  the  units  digit  in  the  special  call  number  is  "  1,"  P-l  will  rest  on  an 
open  contact,  but  P-2  will  rest  on  a  contact  wires  to  the  interrupter 
(ground).  At  the  same  time,  the  line  wipers  rest  on  contacts  wired  to 
the  16-  and  33-cycle  generators.  Hence  the  negative  line  —  L  is  in  a 
position  to  be  connected  to  the  ringing  generators,  and  the  positive  line 
+L  can  be  connectedto  the  ring  cut-off  relay  RCOR  and  negative  battery. 

As  the  interrupter  acts,  the  frequency  relay  will  pull  up  and  fall 
back,  sending  out  alternately  the  two  frequencies  mentioned.  But  the 
subscriber  line  must  be  connected. 

The  first  bit  of  ringing  current  finds  a  path  through  the  starting 
relay  StRy,  200-ohm  winding.  It  pulls  up  its  locking  contact  N  first, 
which  enables  it  to  cut  off  the  initial  circuit.  This  relay  switches  the  im- 
pulsing circuit  of  the  line  relay  from  the  magnets  to  the  switching  relay 
SwRy. 

When  the  calling  subscriber  hangs  up  his  receiver,  the  line  relay  falls 
back.  The  release  relay  will  not  fall  back  at  once,  because  it  is  slow  to 
release.     The  ground  at  the  line  relay  causes  current  to  flow  through 


98  AUTOMATIC  TELEPHONY 

the  front  contact  of  the  starting  relay  to  the  switching  relay.  The  latter, 
pulling  up,  switches  the  lines  from  the  line  relay  to  the  ringing  generators 
which  have  been  prepared.  It  also  connects  the  release  relay  to  a  ground 
which  was  prepared  by  the  off-normal  springs  (through  the  back  contact 
of  the  ring  cut-off  relay). 

Now  the  action  of  the  ringing  interrupter  causes  the  frequency  relay 
to  switch  from  one  frequency  to  the  other,  ringing  the  bell  of  the  called 
station  and  that  of  the  calling  station. 

When  the  called  station  answers,  the  switching  of  the  talking  appa- 
ratus of  the  telephone  across  the  line  permits  the  ring  cut-off  relay  RCOR 
to  energize  (by  direct  current)  and  to  lock  itself  by  its  1300-ohm  winding 
to  the  off-normal  spring  ground.  This  relay  cuts  off  the  ringing  current, 
opens  the  circuit  of  the  release  relay,  and  energizes  the  release  magnet. 
The  reverting  call  switch  at  once  restores  itself  to  normal,  at  which  point 
the  off-normal  springs  cut  the  current  from  the  release  magnet. 

The  falling  back  of  the  release  relay  removes  the  ground  from  the 
release  trunk  so  that  the  selectors  can  release.  It  also  releases  the  start- 
ing relay  StRy  and  the  switching  relay  SwRy. 

Since  there  is  at  least  one  telephone  talking  set  across  the  line,  the 
release  of  the  switches  is  only  momentary.  At  once  the  lineswitch  will 
seize  the  same  trunk  or  another  trunk  so  that  the  line  will  draw  battery 
current  from  a  first  selector  during  conversation. 

If  the  units  digit  of  the  special  call  number  had  been  "2,"  the  fre- 
quency relay  would  have  been  used  as  before,  but  a  different  pair  of 
frequencies  would  have'  been  selected  by  the  line  wipers. 

If  the  units  digit  is  "3,"  the- directing  relay  alone  would  be  operated, 
and  but  one  frequency  selected.  This  frequency  would  be  directed  alter- 
nately to  one  line  wire  and  then  to  the  other.  This  would  ring  a  bell 
from  each  side  of  the  line  to  earth. 

If  the  units  digit  is  "4,"  both  the  frequency  relay  and  the  directing 
relay  are  operated  at  the  same  time  This  rings  one  frequency  on  one 
line  and  another  frequency  on  the  other  line. 

For  a  party  line  which  has  all  bells  bridged,  the  directing  relay  is  not 
used — the  bank  contacts  for  the  P-l  wiper  are  left  open.  The  line  bank 
is  wired  with  the  pairs  of  frequencies  needed  for  all  subscribers  on  a  line 
to  ring  each  other.     The  frequency  relay  alone  is  enough 

For  a  party  line  which  has  bells  bridged  from  each  line  to  earth,  both 
relays  must  be  used  at  times,  one  FR  to  switch  frequencies,  and  the  other 
DrRy  to  direct  the  ringing  current  to  the  desired  line  wire. 

Group  Connectors  or  Rotary  Connectors. — One  subscriber  may  have 
more  than  one  line.  His  lines  may  terminate  on  his  premises  in  as  many 
separate  telephones;  or  on  bells,  with  keys  to  permit  any  telephone  to 
answer  any  line;  or  on  a  private  branch  exchange  (manual)  board;  or  in 
a  private  automatic  exchange. 


AUTOMATIC  ELECTRIC  COMPANY'S  APPARATUS 


99 


Usually  only  one  call  number  is  listed  in  the  directory.  If  this  line 
is  busy,  the  others  will  serve  as  well,  and  must  be  tested,  selected,  and 
used. 

The  group  connector  is  arranged  to  perform  the  above  duties.  It 
may  operate  with  the  customary  two  digits,  being  called  a  "2-digit  group 
connector"  or  "rotary  connector,"  to  distinguish  it  from  the  ordinary 
"line  connector"  which  connects  only  with  single  lines.  It  may  operate 
with  one  digit,  in  which  case  it  is  called  a  "one-digit  group  connector." 
In  this  case  the  group  of  lines  may  occupy  one  level  or  more  than  one 
level. 


Bu5y  Tone 


Mse  Trk. 


RCOR 
Ii00"> 


2M.F. 

Fig.   S4. — Circuit  of  2-digit  group  connector. 


Two -Digit  Group  Connector. — The  circuit  of  a  two-digit  group  con- 
nector is  shown  in  Fig.  84.  There  are  two  private  wipers,  and  the  private 
bank  has  a  pair  of  contacts  for  each  point  (like  line  bank).  P-\  is  the 
upper  wiper  and  P-2  the  lower  wiper,  each  engaging  the  respective  con- 
tact of  the  pair. 

Each  level  may  be  divided  up  into  a  number  of  groups  of  lines,  or 
may  be  used  as  separate  lines.  The  grouping  is  done  on  the  private 
bank.  The  two  contacts  of  a  pair  are  connected  together  for  all  the 
points  except  the  last  of  the  group.     They  are  left  open. 

The  lower  contact  of  each  pair,  engaged  by  P-2,  is  the  usual  private 
contact  and  is  connected  to  the  BCO  of  the  lineswitch  of  the  called  line. 
The  private  wiper  P-2  is  likewise  the  usual  busy-test  wiper  of  the  con- 
nector described  in  Fig.  60. 


100  AUTOMATIC  TELEPHONY 

The  upper  contact  of  each  pair  has  to  do  only  with  hunting  an  idle 
line  in  the  group.  The  private  wiper  P-l  is  used  in  connection  with 
the  rotary  magnet  and  the  rotary- interrupter  relay,  very  much  as  was 
described  in  connection  with  the  selector  of  Fig.  63.  Any  line  which  is 
busy  will  have  a  ground  on  its  lower  private  contact.  (See  Fig.  84.)  It  is 
the  ground  which  operates  the  BCO  of  the  called  line  and  also  insures 
privacy.  This  ground  is  transferred  by  the  tie  wire  to  the  upper  private 
contact,  so  that  while  the  switch  is  hunting,  the  private  wiper  P-l  will 
find  ground  and  cause  the  rotary  magnet  to  drive  on. 

When  an  idle  line  is  found,  both  contacts  will  have  no  ground. 

If  all  lines  in  the  group  are  busy,  the  switch  will  arrive  at  the  last 
pair  of  contacts  in  the  group.  Here  the  wiper  P-l  will  find  no  ground, 
because  the  contact  is  open.  The  control  now  passes  to  the  lower  wiper, 
P-2,  which  tests  the  line  as  usual,  and  gives  the  busy  tone  on  finding  it 
engaged. 

When  the  switch  is  seized,  the  line  relay  and  the  release  relay  RlRy 
pull  up.  The  latter  grounds  the  release  trunk,  prepares  the  impulsing 
circuit  for  the  line  relay,  and  grounds  a  very  useful  conductor  known  as 
the  " release  relay  ground."  The  last-named  wire  enables  the  wiper- 
closing  relay  WCRy  and  the  ring  cut-off  relay  RCOR  to  lock  themselves. 
Through  it  the  rotary  magnet  gets  current  while  hunting  an  idle  line. 

The  hundreds  digit  of  the  call  number  operates  the  vertical  magnet 
in  the  usual  way.  The  series  relay  Se  is  placed  next  to  negative  battery 
so  that  it  can  later  be  used  in  the  rotary  test  circuit.  During  the  vertical 
action,  it  holds  the  vertical  magnet  circuit,  even  though  the  off-normal 
springs  ONS  have  switched  to  the  rotary  magnet. 

At  the  end  of  the  hundreds  digit,  the  series  relay  falls  back  and  com- 
pletes the  switching  of  the  impulsing  circuit  from  the  vertical  magnet 
VM  to  the  rotary  magnet  RM. 

The  units  digit  now  causes  the  rotary  magnet  to  move  the  wipers 
over  the  chosen  level  of  bank  contacts  to  a  definite  set  of  them.  During 
this  time  the  rotary  series  relay  RSe  remains  energized,  cuts  off  the  private 
wiper  P-l,  switches  the  busy  relay  ByRy  onto  P2,  and  insures  the  rotary 
magnet  circuit  even  though  the  busy  relay  pulls  up  while  P-2  is  passing 
over  busy  contacts. 

When  the  units  digit  ceases  its  impulses,  the  rotary  series  relay  falls 
back.  This  closes  a  gap  in  the  P-l  circuit,  switches  P-2  onto  the  125 
pull-up  winding  of  the  wiper-closing  relay,  switches  the  busy-relay  wind- 
ing onto  its  own  front  contact,  and  opens  the  initial  circuit  of  the  rotary 
magnet. 

Assume  that  the  line  is  free.  There  will  be  no  ground  on  either 
private  wiper.  Therefore  there  will  be  a  path  from  ground  on  the 
release  trunk  through  a  back  contact  on  the  series  relay,  back  contact  of 
the  busy  relay,  125-ohm  winding  of  the  wiper-closing  relay,  back  con- 


AUTOMATIC  ELECTRIC  COMPANY'S  APPARATUS  101 

tact  of  rotary-series  relay,  P-2,  BCO  of  called  line  to  negative  battery. 
The  latter  will  pull  up  and  clear  the  called  line. 

The  wiper-closing  relay  will  now  energize,  lock  itself  to  the  release 
relay  ground,  open  the  rotary  magnet  and  the  busy-relay  circuits,  ground 
the  private  P-2,  and  connect  the  line  wipers  to  the  ringing  current  sup- 
plies through  the  contacts  of  the  ring  cut-off  relay. 

Ringing  current  now  flows  from  the  generator  through  a  resistance 
lamp,  the  200-ohm  winding  of  the  ring  cut-off  relay,  a  back  contact  on 
the  same  relay,  through  the  negative  wiper  —  W  to  the  line,  through  the 
bell  and  condenser  in  the  telephone  back  over  the  positive  line  to  ground 
at  the  ring  cut-off  relay.  A  little  current  passes  through  the  0.04  MF 
condenser  to  the  calling  line  to  indicate  that  ringing  is  being  done. 

When  the  called  station  answers,  the  ring  cut-off  relay  operates  and 
locks  itself  to  the  release  relay  ground,  and  switches  the  lines  from  the 
generator  to  the  calling  subscriber  lines  and  the  back-bridge  relay  BBR. 
The  latter  supplies  direct  current  to  the  called  station,  reverses  the  cur- 
rent flow  to  the  calling  subscriber  line,  and  prepares  a  circuit  for  super- 
visory signal  No.  2  (*S-2). 

This  switch  is  arranged  to  release  when  the  last  party  hangs  up.  The 
details  need  not  be  repeated  here. 

If  a  few  lines  are  busy,  the  special  features  of  the  switch  are  revealed. 
Assume  that  the  second  series  of  impulses  has  just  ceased  and  that  the 
rotary  series  relay  RSe  is  about  to  let  go.  At  this  time  the  busy  relay 
ByRy  will  be  energized,  because  there  is  a  ground  on  P-2  and  the  rotary 
series  relay  is  holding  the  busy  relay  onto  that  wiper.  Since  the  busy 
relay  is  energized,  the  rotary-interrupter  relay  RIR  circuit  is  prepared 
and  the  initial  circuit  of  the  rotary  magnet  is  opened. 

When  the  rotary-series  relay  falls  back,  it  switches  the  busy  relay 
onto  the  release  trunk,  where  it  locks  itself.  The  rotary-series  relay 
also  closes  the  circuit  of  the  rotary-interrupter  relay  to  the  wiper  P-l. 

We  now  have  three  circuits  mutually  interconnected.  The  series 
relay  Se  and  the  rotary-interrupter  relay  RIR  are  in  one  circuit  to  P-l 
and  are  affected  by  the  rotary  magnet  RM  and  the  busy  relay  ByRy. 
The  rotary  magnet  is  in  another  circuit  and  is  controlled  by  the  rotary- 
interrupter  relay.  The  busy  relay  is  in  still  another  circuit,  locked 
through  the  series  relay  contacts. 

As  soon  as  the  rotary  series  relay  falls  back,  closing  the  P-l  circuit, 
the  series  relay  and  the  rotary-interrupter  relay  pull  up.  The  former 
transfers  the  busj^-relay  circuit  from  the  release  trunk  to  P-2,  so  that  its 
continued  energization  depends  upon  busy  lines.  The  latter  pulls  up 
the  rotary  magnet. 

The  rotary  magnet  rotates  the  wipers  onto  the  next  set  of  contacts. 
If  they  also  are  grounded,  the  busy  relay  will  remain  energized.  Near 
the  end  of  its  stroke,  the  rotary  magnet  opens  the  P-l  circuit.     The 


102  AUTOMATIC  TELEPHONY 

series  relay  holds  on,  but  the  rotary-interrupter  relay  falls  back  and  opens 
the  circuit  of  the  rotary  magnet.  The  latter  falls  back  and  closes  the  P-l 
circuit  so  that  the  rotary-interrupter  relay  can  test  the  line  upon  which 
the  wipers  are  now  resting. 

In  this  way  the  rotary  magnet  and  the  rotary-interrupter  relay  play 
back  and  forth,  driving  the  wipers  over  the  group  of  line  contacts. 

As  soon  as  an  idle  line  is  found,  both  P-l  and  P-2  will  fail  to  find 
ground.  The  busy  relay  will  release.  The  rotary-interrupter  relay  will 
not  be  able  to  pull  up.     The  rotation  stops. 

Now  the  series  relay,  failing  to  get  pulsations,  falls  back  and  grounds 
the  P-2  circuit  through  the  125-ohm  winding  of  the  wiper-closing  relay. 
The  latter  now  pulls  up  and  seizes  the  called  line  as  was  described. 

If  all  the  lines  in  the  group  are  busy,  the  rotation  will  continue  until 
the  wipers  are  resting  on  the  last  set  of  bank  contacts.  Here  the  P-l 
wiper  finds  no  ground  but  the  P-2  wiper  rests  on  ground.  The  rotary- 
interrupter  relay  will  not  pull  up,  so  that  the  rotary  magnet  can  not  get 
a  circuit  and  the  rotation  stops.  But  since  P-2  is  grounded,  the  busy 
relay  will  be  energized.  When  the  series  relay  falls  back,  it  transfers 
the  busy-relay  locking  circuit  to  the  release  trunk. 

In  this  condition  the  wiper-closing  relay  can  not  operate  and  the 
busy  tone  is  connected  to  the  line. 

When  the  calling  subscriber  hangs  up  under  this  condition,  further 
rotation  is  prevented  by  the  busy  relay,  which  holds  the  rotary  magnet 
circuit  open. 

One-Digit  Group  Connector. — If  the  number  of  subscriber  lines  in  a 
group  is  greater  than  ten,  more  than  one  level  must  be  set  aside  as  one 
group.     The  one-digit  group  connector  (Fig.  85)  enables  this  to  be  done. 

The  bank  has  a  vertical  part  which  is  attached  to  the  regular  bank 
at  the  right  side.  There  is  a  vertical  wiper  VW  attached  to  the  wiper 
shaft  in  such  a  way  that  it  will  wipe  over  the  contacts  of  the  vertical 
bank,  but  when  the  shaft  starts  to  rotate,  the  vertical  wiper  will  swing 
away  from  the  vertical  bank. 

This  switch  requires  only  one  digit  to  operate  it.  This  one  group  of 
impulses  lifts  the  shaft  to  the  desired  level,  after  which  the  operation  is 
taken  out  of  the  subscriber's  hands.  The  switch  tests  the  level  at  which 
the  group  of  impulses  delivered  it.  If  the  level  has  one  or  more  lines 
free,  the  switch  will  rotate  and  find  one.  If  all  lines  on  that  level  are 
busy,  the  vertical  wiper  will  discover  it  and  cause  the  switch  to  lift  the 
wipers  to  the  next  level  where  the  test  is  repeated.  In  this  way  an  idle 
level  and  an  idle  line  are  found.  If  all  lines  are  busy,  the  wipers  rotate 
off  the  bank  at  the  top  level  of  the  group,  close  springs  11-P,  and  give 
the  busy  tone. 

Attached  to  each  private  contact  in  a  group  of  lines  is  a  busy  relay 
known  as  a  horizontal-chain  relay.     Their  contacts  are  connected  in 


AUTOMATIC  ELECTRIC  COMPANY'S  APPARATUS 


103 


series,  so  that  when  all  the  lines  of  the  level  are  busy,  they  cause  the 
energization  of  the  corresponding  vertical-chain  relay.  The  vertical- 
chain  relay  grounds  the  vertical-bank  contact  which  corresponds  to  the 
level  which  is  busy  and  on  which  the  vertical  wiper  rests  when  the  line 
wipers  stand  opposite  that  level. 

The  vertical  wiper  VW  is  connected  to  negative  battery  through  the 
500-ohm  winding  of  the  vertical  relay  YRy.  This  is  the  vertical  testing 
circuit. 

When  the  switch  is  seized,  the  line  relay  LR  pulls  up  and  energizes 
the  release  relay  RlRy.     The  latter  grounds  the  release  trunk,  prepares 


VRu 
•'H«|l[l — &.500" 


Vertical 
Bank 


VM  ^ 


Rise  Trk 


I800c 


~X-TT 


yw°- 


Private  Bank 


Rmglntr'Sfarf 
Fig.   85. — Circuit  of  1-digit  group  connertor. 


the  circuit  for  the  vertical  magnet,  and  grounds  a  wire  known  as  the 
"release-relay  ground"  RlRyGrd. 

While  the  subscriber  is  letting  the  dial  rotate  back  on  the  digit,  the 
line  relay  vibrates  and  sends  impulses  to  the  vertical  magnet  through  the 
series  relay  Se.  The  former  lifts  the  shaft,  operating  the  off-normal 
springs  ONS  The  series  relay  connects  the  interrupter  relay  IRy  to 
earth  as  follows:  through  off -normal  springs  ONS,  front  contact  of  series 
relay,  back  contact  of  wiper-closing  relay  WCRy,  to  release  relay  ground 
RlRyGrd.  The  interrupter  relay  pulls  up,  short  circuiting  the  1300-ohm 
winding  of  the  wiper-closing  relay  and  preparing  a  circuit  for  the  rotary 
magnet. 

During  the  vertical  motion,  the  vertical  wiper  ITT*  has  been  wiping 


104  AUTOMATIC  TELEPHONY 

over  the  vertical  bank.  Whenever  it  found  a  ground,  VRy  pulled  up, 
when  it  found  no  ground  it  fell  back.  Its  condition  at  the  time  of  stop- 
ping depends  upon  the  condition  of  the  level.  Assume  the  latter  to  be 
busy,  and  the  vertical  contact  grounded.  The  vertical  relay  therefore  is 
energized.  It  assists  the  interrupter  relay  in  short  circuiting  the  1300- 
ohm  winding  of  the  wiper-closing  relay  but  diverts  the  circuit  prepared 
for  the  rotary  magnet  to  the  vertical  magnet. 

At  the  end  of  the  series  of  impulses  on  the  line  relay,  the  series  relay 
falls  back,  switching  the  release  relay  ground  to  the  circuit  prepared  by 
the  interrupter  relay  and  the  vertical  relay.  This  circuit  runs  as  follows : 
release-relay  ground,  back  contact  of  wiper-closing  relay,  back-contact 
series  relay,  front-contact  interrupter  relay,  front-contact  vertical  relay, 
vertical  magnet  VM,  to  negative  battery.  The  interrupter  relay  now 
is  held  energized  through  ONS,  back-contact  RM,  back-contact  VM, 
front-contact  IRy  to  release-relay  ground. 

The  vertical  magnet  now  pulls  up  and  lifts  the  shaft  to  the  next 
level,  which  the  vertical  relay  tests.  Assume  that  a  number  of  the  levels 
are  busy,  so  that  we  may  get  the  action  of  the  apparatus.  If  the  level 
now  tested  is  busy,  the  vertical  relay  will  remain  energized,  so  that  the 
1300-ohm  winding  of  the  wiper-closing  relay  will  be  short-circuited  re- 
gardless of  the  interrupter  relay. 

Near  the  end  of  its  stroke,  the  vertical  magnet  inserts  1800  ohms 
into  the  circuit  of  the  interrupter  relay.  The  latter  falls  back,  opening 
the  circuit  of  the  vertical  magnet.  Then  the  vertical  magnet  falls  back 
and  closes  the  circuit  of  the  interrupter  relay.  As  long  as  the  vertical 
wiper  finds  ground  and  the  vertical  relay  therefore  remains  energized, 
this  mutual  vibration  of  vertical  magnet  and  interrupter  relay  continues. 
The  short  pulsations  through  the  1800-ohm  winding  of  the  vertical  relay 
keep  it  energized  and  prevent  premature  rotation. 

When  an  idle  level  is  found,  the  vertical  relay's  500-ohm  winding 
ceases  to  pull.  Near  the  end  of  the  vertical-magnet  stroke  which  brings 
the  wipers  to  this  level,  the  vertical  magnet  takes  the  short  circuit  off 
the  1800-ohm  winding  of  the  vertical  relay.  The  interrupter  relay  falls 
back  and  causes  the  vertical  magnet  to  do  the  same. 

The  1800-ohm  winding  of  the  vertical  relay  holds  it  energized  a  little 
longer  than  it  would  otherwise  be.  On  the  release  of  the  vertical  magnet 
the  interrupter  relay  pulls  up.  The  vertical  relay  now  releases,  switch- 
ing the  circuit  from  the  vertical  magnet  to  the  rotary  magnet  RM. 
The  latter  at  once  rotates  the  wipers  onto  the  first  set  of  contacts  in  the 
level.     The  private  wiper  tests  the  private  contact  for  ground. 

Assume  that  the  first  few  lines  are  busy.  The  private  wiper  then 
rests  on  ground,  so  that  the  wiper-closing  relay  cannot  act.  The  mutual 
vibration  above  described  is  now  repeated,  between  the  rotary  magnet 
and  the  interrupter  relay. 


AUTOMATIC  ELECTRIC  COMPANY'S  APPARATUS  105 

When  the  private  wiper  arrives  at  a  free  (ungrounded)  contact,  the 
interrupter  relay  will  be  unable  to  pull  up  because  of  the  1300-ohm  wind- 
ing of  the  wiper-closing  relay.  But  the  latter  WCRy  will  energize  and 
perform  the-  following  acts:  Switch  the  private  wiper  from  test  circuit 
to  ground,  to  make  busy  the  called  line  and  to  operate  its  bridge  cut-off 
relay  BCO  to  clear  the  line  of  attachments,  switch  the  release-relay  ground 
RlRrjGrd  from  the  magnets  to  the  release  trunk,  ground  the  ringing 
interrupter-start  wire,  insert  150  ohms  into  vertical-magnet  circuit,  and 
close  line  wipers  to  ringing  current  through  the  springs  of  the  ring  cut-off 
relay  RCOR. 

Ringing  now  takes  place  by  means  of  current  from  the  ringing  gener- 
ator RiGen  through  the  200-ohm  winding  of  the  ring  cut-off  relay  and 
battery.  The  ring  cut-off  relay  is  slow  acting  and  will  not  respond  to 
alternating  current  of  the  lowest  frequency  used.  It  acts  only  with 
battery  current.  The  ringing  interrupter  Rilntr  which  was  started  by 
the  wiper-closing  relay  now  alternately  cuts  the  generator  into  and  out 
of  circuit.  When  the  generator  is  out  of  circuit,  the  positive  wiper  +  W 
is  connected  directly  to  earth. 

When  the  called  station  answers,  the  direct  current  drawn  from 
battery  through  the  telephone  set  operates  the  ring  cut-off  relay,  which 
locks  its  1300-ohm  winding  to  the  release-relay  ground.  At  this  time 
the  release  trunk  and  the  release-relay  ground  are  connected  together. 
The  same  action  of  the  ring  cut-off  relay  furnishes  ground  for  the  back- 
bridge  relay  BBR,  so  that  the  called  station  will  get  battery  current  for 
talking. 

When  the  back-bridge  relay  operates,  it  reverses  the  current  which  is 
flowing  to  the  calling  station,  takes  control  of  the  release  magnet  circuit, 
and  completes  a  ground  connection  on  the  release-relay  ground  RlRYGrd 
through  contacts  on  the  ring  cut-off  relay  and  the  wiper-closing  relay. 
At  this  time  the  combined  release  trunk  and  RlRyGrd  have  three  grounds 
on  them,  two  on  the  release  relay  (separate  contacts)  and  one  through 
the  WCRy,  RCOR,  and  BBR. 

The  action  during  conversation  and  release  is  as  was  described  for 
Fig.  60.  The  connector  will  not  be  released  until  both  parties  have 
hung  up  their  receivers.  If  the  calling  station  hangs  up  first,  the  ground 
on  the  release  trunk  will  be  removed  long  enough  for  the  selectors  and 
lineswitches  to  release,  then  it  will  be  replaced.  This  is  accomplished  by 
the  series  relay  Se  which  pulls  up  momentarily  between  the  release  of  the 
line  relay  and  that  of  the  release  relay.  The  150  ohms  which  was  inserted 
by  the  wiper-closing  relay  prevents  the  vertical  magnet  from  acting,  but 
permits  the  series  relay  to  energize  for  that  purpose.  By  the  time  that 
the  series  relay  has  pulled  up  and  fallen  back,  the  selectors  and  line- 
switches  have  released  and  the  release  relay  of  the  connector  has  let  go, 


10G  AUTOMATIC  TELEPHONY 

It  is  now  the  BBR,  RCOR,  and  WCRy  that  mutually  hold  each  other  and 
place  a  ground  on  the  release  trunk  so  as  to  busy  the  switch. 

If  the  first  level  of  the  group  has  one  idle  line,  there  will  be  no  ground 
on  the  first  contact  of  the  vertical  bank.  Therefore  the  vertical  relay 
will  not  act  again,  and  the  rotary  motion  begins  as  soon  as  the  series 
relay  falls  back. 

If  all  lines  of  all  levels  are  busy,  the  switch  will  run  up  to  the  top  level 
and  rotate  past  the  tenth  set  of  contacts  and  off  the  bank  (position  11-R) 
where  it  will  give  the  busy  tone  to  the  calling  subscriber.  This  is  done  by 
leaving  the  vertical-bank  contact  for  the  top  level  of  the  group  unattached 
to  anything. 

The  set  of  springs  marked  "11-R"  operates  when  the  wiper  shaft  ro- 
tates off  the  tenth  set  of  contacts  of  any  level.  One  part  of  the  set 
grounds  the  1300-ohm  winding  of  the  ring  cut-off  relay  to  cause  it  to  pull 
up  and  lock,  the  other  part  switches  busy  tone  into  the  positive  wind- 
ing of  the  line  relay,  by  which  device  the  calling  subscriber  hears  the 
tone. 


CHAPTER  IV 
TRUNKING,  ITS  PHYSICAL  ARRANGEMENTS  AND  VARIATIONS 

Automatic  apparatus  may  be  mounted  and  installed  in  a  variety  of 
ways.     A   number  of   combinations   have   been  used   during  the   past 


Fig.  86. — One-hundred-line  switchboard, 
Keith  lineswitches  (front). 


Fig.   87. — One-hundred-line  switchboard, 
Keith  lineswitches  (back). 


twenty-seven  years  and  new  arrangements  are  still  being  considered. 
Each  has  its  own  advantages,  and  an  engineer  can  lay  out  a  system 
with  the  assurance  that  the  equipment  can  be  moulded  in  such  a  way  as 

107 


108 


A  I'TOMA  TI(  ■   TELEPHONY 


fully  to  meet  the  needs  of  conditions.  Flexibility  is  characteristic  of 
automatic  apparatus. 

It  is  the  purpose  of  this  chapter  to  show  the  most  approved  ways  of 
mounting  automatic  switches  on  frames  for  installation,  to  give  an  idea 
of  how  the  switches  are  linked  together  in  practice,  and  to  illustrate  the 
flexibilities  in  trunking  arrangements. 

Lineswitches  and  Connectors. — The  prevailing  practice  is  to  mount 
lineswitches  on  one  side  of  a  frame,  with  the  connectors  on  the  other 
side.     (See    Fig.  86.)     The  Keith  lineswitches  here  shown  are  in  four 


k  b  b' k  k  b  b  l»"l» 


imXTD 


^mmm 


b  t?  in  Is  b  .b  b  b  tn  i? 


'tm\mimimmMk»tmim*lK 


W 


Fig.  88. — One-hundred-line  switchboard,  rotary  lineswitches  (front). 

groups  of  25  switches  per  group.  The  two  at  the  left  (50  lines)  are 
mounted  on  one  shelf,  which  is  hinged  at  the  left.  It  may  be  swung 
out  to  give  access  to  the  wiring  on  its  back.  The  right  shelf  carrying 
fifty  lineswitches  is  hinged  at  the  right  so  as  to  swing  out  also. 

As  shown  there  is  one  master  switch  for  each  shelf  of  50  lineswitches. 
Hence  these  50  lines  trunk  into  ten  trunks.  Such  an  association  is 
called  a  "section."  If  the  traffic  is  light,  all  of  the  hundred  lineswitches 
may  trunk  into  the  same  ten  trunks,  be  governed  by  one  master  switch, 


TRUNKING,  ITS  PHYSICAL  ARRANGEMENTS  AND  VARIATIONS     109 

and  be  termed  one  section.  If  the  traffic  is  heavy,  each  shelf  may  be 
provided  with  two  master  switches,  one  for  each  25  lineswitches.  In 
this  case  25  lines  with  their  master  switch  constitute  a  section. 

At  the  very  top  of  the  frame  is  a  small  powerboard,  with  bus  bars 
for  the  battery,  and  fuses  to  protect  the  apparatus  and  wiring. 

Just  below  the  power  panel  is  the  terminal  assembly.  Here  are 
a  number  of  strips  each  containing  100  terminals,  to  which  are  connected 
the  various  pieces  of  apparatus.  It  is  the  meeting  point  for  circuits, 
useful  for  interconnections  and  testings. 


Fig.   89. — One-hundred-line  switchboard,  rotary  lineswitches  (front  open). 


On  the  back  of  the  frame  (Fig.  87)  are  the  connectors  which  carry 
traffic  into  the  100-line  group.  Four  shelves  are  provided,  so  that  any 
number  of  connectors  may  be  installed  up  to  24.  Whenever  a  connector 
is  removed,  its  bank  is  left  behind,  together  with  its  bank  rods.  If  the 
removal  is  to  last  for  some  time,  or  banks  are  installed  without  switches, 
the  bank  is  held  by  a  support  which  is  bolted  onto  the  connector  shelf. 

A  100-line  switchboard  with  rotary  lineswitches  (Fig.  88)  is  also 
arranged  with  two  shelves.  Each  shelf  has  50  lineswitches  arranged  in 
five  horizontal  rows.     The  line  relay  and  the  cut-off  relay  of  each  switch  is 


110 


AUTOMATIC  TELEPHONY 


protected  by  a  cover  to  keep  out  the  dust.     The  second  row  is  shown 
with  the  covers  removed. 


Fig.  90. — One-hundred-line  switchboard,  rotary  lineswitches  (back). 

r9   ?   ?   9   9    ? 


rt.P.ti    i    i     i    l    i    I    i    i 


Connectors 


LincLP.F) 


l.Sw.Bd. 


To  Selectors  or 
Secondary  L .  Sw. 

Fig.  91. — Line  intermediate  distributing  frame. 

The  two  shelves  are  hinged  to  swing  outwardly.  (Fig.  89.)  This 
exposes  the  wiring  and  the  condensers  of  the  connectors.  At  the  top  is 
the  terminal  assembly. 


TRUNKING,  ITS  PHYSICAL  ARRANGEMENTS  AND  VARIATIONS     111 

The  back  of  the  frame  (Fig.  90)  supports  the  connectors  on  three 
shelves  of  eight  switches  each.  Three  of  the  connectors  have  their 
covers  removed.  The  one  at  the  extreme  right  of  the  bottom  shelf  is  a 
test  connector,  used  by  a  tester  at  the  wire  chief's  desk.  One  connector 
is  not  installed,  the  bank  being  held  by  a  support. 

At  the  left  are  three  little  fuse  panels,  one  for  each  shelf.  There  is  one 
fuse  per  connector.  At  the  top  is  another  fuse  panel  with  one  fuse  for 
each  ten  lineswitches. 


Fig.  92. — Connector  trunk  shelf  (front). 

Lineswitches  and  Connectors  Separate. — In  recent  years  some  instal- 
lations have  been  made  in  which  the  lineswitches  have  been  treated  as 
separate  from  the  connectors.  Each  line  is  connected  permanently  to  the 
assigned  place  on  the  connector  banks,  corresponding  to  the  call  number. 
This  is  for  outgoing  calls  and  will  not  be  changed  as  long  as  the  call 
number  remains  constant.  In  addition  the  subscriber  line  is  connected  to 
a  lineswitch  This  is  for  incoming  calls  to  the  central  office  and  requires 
only  that  sufficient  trunking  facilities  be  provided.     The  grouping  may  be 


iiWraSiaky  B 


1 

'Tiiirrtirt 

b."  ■■¥sss7*i!n        :i  v;::      i .         y. 

mva js.— -a     — — ^ 

Fig.  93. — Connector  trunk  shelf  (back). 

larger  than  100  lines.  An  intermediate  distributing  frame  is  provided 
between  the  lines  and  the  lineswitches,  so  that  any  line  may  be  connected 
to  any  lineswitch.  This  is  very  much  like  manual  practice  in  a  multiple 
switchboard. 

The  general  arrangement  is  shown  in  Fig.  91.  The  subscriber  lines 
arrive  on  the  vertical  side  of  the  main  distributing  frame  MDF  at  the 
protectors.  From  the  horizontal  side  of  the  frame  the  lines  are  cabled 
to  the  horizontal  side  of  the  line  intermediate  distributing  frame  (line 


112 


A  UTOMA  TIC  TELEPHON  Y 


IDF)  where  cables  run  to  the  terminal  assembly  on  the  connector  frame 
and  thence  to  the  connector  banks. 

On  the  line  IDF  the  lines  are  cross  connected  to  the  vertical  side 
where  cables  lead  to  the  lineswitch  board  frames.  From  the  banks  of 
the  lineswitches  the  trunks  run  to  first  selectors  or  to  secondary  line- 
switches. 

The  lineswitches  are  mounted  on  the  usual  frames,  but  occupy  both 
sides  of  them.  The  connectors  are  mounted  on  shelves  much  like  selector 
shelves.  (Fig.  92.)  There  is,  however,  a  short  shelf  carrying  five 
switches,  joined  to  the  long  one  (ten  switches)  making  fifteen  switches  in 
all.     The  bank  terminal  boards,  fuses,  and  relays  are  between.     The 


Fiu.  94. — Connector  trunk  shelf,  power  panel  and  bank  terminals. 


back  side  (Fig.  93)  carries  the  condensers.  Five  or  six  shelves  are  built 
into  a  frame.  The  battery  bus  bars  are  connected  vertically.  An 
enlarged  view  of  the  bank  terminal  boards,  fuses  and  relays  is  shown  in 
Fig.  94. 

Local  Secondary  Lineswitches. — Local  secondary  lineswitches  are 
mounted  on  frames  like  those  used  for  the  primary  lineswitches,  both 
sides  of  the  frame  being  occupied  by  switches.     (Fig.  95.) 

Between  the  primary  lineswitches  and  the  secondaries  is  an  inter- 
mediate distributing  frame  (primary  IDF)  on  which  are  made  the  inter- 
connections necessary  to  scatter  the  traffic,  as  described  in  Chapter  2. 

Between  the  secondary  lineswitches  and  the  first  selectors  is  another 
frame  (secondary  IDF)  (Fig.  96)  for  the  purpose  of  distributing  the 
traffic  among  the  sections.     The  general  plan  is  shown  by  the   figure. 


TRUNKING,  ITS  PHYSICAL  ARRANGEMENTS  AND  VARIATIONS     113 

The  secondary  lineswitches  are  divided  into  groups  (Fig.  97)  and  the 
first  selectors  into  bays.  The  secondary  IDF  accomplishes  the  distribu- 
tion of  traffic  shown  on  the  drawing.  In  a  full  sized  installation,  there 
are  ten  sections  of  secondary  lineswitches  in  each  group.  Since  each 
section  has  ten  outgoing  trunks,  the  group  has  100  trunks.  There  are 
ten  sections  of  first  selectors  (lettered  A  to  J)  in  each  bay.  Since  there 
are  ten  selectors  in  a  section,  the  bay  has  100  first  selectors. 

The  distribution  provides  ten  trunks  from  each  secondary  lineswitch 
group  into  each  bay  of  selectors.  Within  a  given  group  each  section 
delivers  all  of  its  ten  trunks  to  a  given  selector  bay.  Thus  all  the  sections 
numbered  "1"  trunk  to  bay  No.  1,  all  the  sections  numbered  "2"  trunk 


Fig.  95. — Secondary  lineswitches  installed. 


to  bay  No.  2,  etc.  Within  the  section  of  lineswitches  the  ten  trunks  are 
distributed  to  all  the  selector  sections  in  the  given  bay.  Thus  the  first 
trunk  goes  to  a  selector  in  section  A,  the  second  trunk  to  a  selector  in 
section  B,  etc. 

The  above  method  secures  an  effective  scattering  of  the  traffic  among 
all  the  trunks  which  leave  the  banks  of  1000  first  selectors,  although  each 
group  of  lineswitches  is  connected  to  only  100  first  selectors. 

Selectors. — Selectors  are  mounted  on  fixed  shelves,  each  of  which 
holds  20  switches.  Fig.  98  shows  a  view  of  mounted  selectors.  Five  or 
six  shelves  are  mounted  on  one  frame  known  as  a  "bay."  Two  bays 
make  up  a  "selector  board."  Three  such  boards  are  shown  in  the 
illustration. 


114 


AUTOMATIC  TELEPHONY 

Sectionl.  Section?  Sachem's    E"tc. 


Secondary 

Line 
Switches 


Secondare/ 
I.D.F.  * 


lOSels.        lOSels.        lOSels     Etc. 
Fig.  96. — Secondary  intermediate  distributing  frame. 


Secondary 
Line  Switches 


First  Selector 
Boards 


Group  A 


Group  B 


Group  C 


Etc 


-Bom  I 


:Bay2 


.Bay  3 


Etc 


Etc. 


There  are  as  many  first  Selector  bays  as  groups  of  Secondary 
Linesw'/fches.  Each/Lines,  'itch  Sect/on  spreads  its  trunks  over 
the  sections  of  only  one  Selector  Board.  All  L.Sw-Sections  I  to 
Bay  I.  All  L.Siv.  Sections  2  to  Bay2jetc.  Each  Selector  Bay  receives 
tru  nhs  from  all  L .  5w.  groups .  Each  Se/ec  tor  Section  receives 
trunks  from  particular  L.Sw.  Bank  Contacts.  A-from-^B-from^Efc. 

Fig.  97. — Trunking  secondary  lineswitches  to  first  selectors. 


TRUNKING,  ITS  PHYSICAL  ARRANGEMENTS  AND  VARIATIONS     115 

The  end  of  the  board  nearest  the  reader  contains  the  terminal  assem- 
bly, covered  by  two  doors.  This  is  a  very  interesting  and  valuable  part 
for  by  means  of  it  all  the  trunking  flexibilities  are  worked  out,  and  a  study 
of  it  will  unlock  the  plan  by  which  any  office  or  exchange  is  linked 
together. 

The  other  end  of  the  selector  board  (Fig.  99)  contains  a  doorway 
to  give  access  to  the  interior  of  the  board,  and  the  fuses,  lamps  and 
relays  used  for  supervisory  purposes. 

The  plan  of  the  board  (Fig.  100)  shows  the  relations  more  clearly. 
The  names  of   the  two  bays  (high  and  low)  were  derived  from  their 


Fig.  98.- — Selectors  installed. 


proximity  to  the  ends  of  the  terminal  assembly.  The  strips  in  the 
latter  are  numbered  from  left  to  right.  Hence,  one  bay  adjoins  the  low 
numbered  end  of  all  strips,  while  the  other  is  near  the  high  numbered 
ends.  It  is  now  customary  to  number  all  the  bays  or  frames  in  a  straight 
row  regardless  of  boards,  and  to  identify  them  by  those  numbers. 

A  development  of  the  selector  board  (Fig.  101)  shows  how  the  selector 
banks  are  wired  to  the  strips  of  terminals.  Usually  the  "A  high" 
section  and  the  "A  low"  section  have  their  banks  wired  to  the  same 
terminal  supports.  Sometimes  the  A  and  the  F  sections  on  the  high  side 
have  their  banks  together. 


116  AUTOMATIC  TELEPHONY 

Terminal  Assembly  for  Selectors. — The  banks  of  the  ten  selectors  of  a 

section  are  multipled  together  and 
cabled  to  sets  of  terminal  strips 
(Fig.  102.)  The  100  wires  from  the 
private  banks  are  fanned  out  as  shown 
above  in  the  illustration.  One  strip 
holds  all  of  them.  The  100  pairs  of 
wires  from  the  line  banks  require  two 
terminal  strips  of  100  points  each. 
The  pairs  from  the  even  numbered 
levels  are  wired  to  one  strip  and  those 
from  the  odd  numbered  strips  to  the 
other  strip,  as  shown  in  the  lower 
part  of  the  figure.  The  cause  of  this 
particular  separation  is  purely  histori- 
cal. The  old  switches  had  two  sepa- 
rate line  banks. 

In  order  for  the  proper  connections 
to  be  made,  these  three  strips  are 
mounted  on  three  separate  supports, 
one   above   the   other.      The   usual 

i  Poor  (Power  Board  Above  Door) 


Lov/S/de 
orBajj-> 


'icfhSide 
£••  or  Bat/ 


'Terminals 


Fig.  99. — Power  end  of  selector  board.  Fig.   100. — Plan  of  selector  board. 

Terminals 


Low  Side  or  Bay 


High  Side  or  Bay 


Fig.    101. — Development  of  selector  board. 

order  from  the  top  downward  is  private,  even  level  lines,  odd  level  lines. 
If  the  A  high  and  A  low  sections  are  to  be   multipled  together,    there 


TRUNKING,  ITS  PHYSICAL  ARRANGEMENTS  AND  VARIATIONS     117 

will  be  the  two  private  strips  on  the  first  support,  the  two  even  level  line 
strips  on  the  second  support,  and  the  two  odd  level  line  strips  on  the 
third  support. 

The  arrangement  is  illustrated  in  Fig.  103,  using  condensed  symbols 
for  the  terminal  strips.     In  this  case,  the  A  and  F  sections  are  multipled. 


Private  Bank 
Contacts 

3i    H    iJ  21   -    !S    I!    ^   L9    —Level-3 

\Level-l 


Private 

Terminal  Strip 


Line  Bank  Contacts 


10'    5/'     32      _,' 
Terminal 
Strip 


+L-L 

(b) 


Sel. 


Fig.   102. — Wiring  of  selector  banks  to  terminal  strips. 

Each  support  requires  at  least  one  terminal  strip  for  each  section 
which  it  carries,  one  strip  for  the  multiple  cable  going  down  to  other  sup- 
ports, one  strip  for  the  multiple  cable  going  up  to  other  supports,  and 
such  other  strips  as  the  variations  in  trunking  require.  They  are  con- 
nected together  by  bending  the  free  ends  and  soldering  them  together. 
This  is  indicated  in  terminal  assembly  drawings  by  vertical  lines  joining 
the  symbols  for  strips. 


118 


A 11  TOM  A  TIC  TELEPHON  Y 


This  illustration  is  based  on  a  multi-office  system.  This  office  is 
No.  4.  The  first  three  levels  are  trunked  out  to  offices  1,  2,  and  3,  re- 
spectively. The  A  and  F  sections  of  the  high  bay  occupy  supports  1,  2 
and  3.  Sections  A  and  F  low  occupy  supports  4,  5,  and  6.  Logically, 
sections  B  and  G  high  will  take  supports  7,  8,  and  9  and  sections  B  and  G 
low  will  take  supports  10,  11  and  12. 


SYMBOLS 


P= Private  Bank  Wires 
E= Even  Levels  \LineBanh 
0=Odd  LevelsK    Wires, 


Support..- 
Numbers^ 


High 
AjSecf/ori 


End  View 


Four+h  Level 


Terminals 
Bent  Together 

Fig.    103. —  Multipling  between  terminals  of  first-selector  banks. 


The  first  three  levels  are  multipled  to  all  the  sections  which  are 
shown  here.  This  is  because  the  outgoing  traffic  is  light  enough  to  war- 
rant this  grouping.  The  fourth  level  is  local;  it  carries  traffic  between 
subscribers  in  this  office.  Accordingly,  more  trunks  are  provided  by 
giving  each  section  or  group  of  sections  its  own  group  of  ten  trunks. 


TRUNKING,  ITS  PHYSICAL  ARRANGEMENTS  AND  VARIATIONS     ll9 

The  above  is  accomplished  by  bending  the  free  ends  of  the  terminals. 
See  support  No.  1,  strip  for  A  high,  level  4  These  terminals  are  bent 
up  and  soldered  to  the  terminals  of  trunks  to  a  section  of  second  selectors. 
The  terminals  for  the  F  high  selectors  are  bent  down  and  soldered  to 
trunks  leading  to  another  section  of  second  selectors.  This  takes  care 
of  the  private  banks. 

The  line  terminals  on  the  second  support  are  treated  in  the  same 
way  as  the  private  terminals.  In  every  case  the  privates  and  the  lines 
are  connected  alike.  Supports  4  and  5  reveal  the  same  method  for 
sections  A  and  F  low. 

A  wide  variety  of  trunk  connections  can  be  effected  by  proper  com- 
binations of  vertical  connections  in  the  terminal  assembly. 

Trunk  Symbols. — The  present  symbol  for  selectors  developed  out  of 
an  older  symbol  which  suggested  the  apparatus  more  nearly  than  does 
the  present  one.     (Fig.  104.)     Ten  trunks  arrive  from  the  left  and  pass 


lOTrunks  • 
/0  Trunks 


Old  Symbol  for  a  Shelf  of  Selectors, 


ftrn/o^^^ =*—IOO  Trunks 


.Jack  of  Swifc.he&. 
10-Trunks  — > — L .  • 

lOTrunks-^  l~jL 


10-Swirches^-- ^      - Q+WSwi+ches 

Banks,       | Banks  ZZ-100-Trunks 

Present  Symbol  for  Selector  Shelf,  Two  Se cf ion s,£0 Switches 
Fig.   104. — Selector  shelf  symbols. 

to  the  jacks  of  a  section  of  ten  switches.  Each  switch  is  represented  by 
a  short  thick  vertical  line.  The  banks  are  indicated  by  half  circles, 
joined  to  show  that  they  are  multipled.  The  switches  in  each  section 
were  numbered  separately. 

In  the  symbol  now  used,  the  general  lines  of  the  older  form  may  still 
be  discerned,  although  the  switches  have  become  a  single  horizontal 
line  and  the  banks  have  been  flattened  into  a  thinner  line.  It  is  now  cus- 
tomary to  number  the  switches  of  a  frame  consecutively  from  1  to  100, 
beginning  with  the  left  hand  switch  on  the  top  shelf  and  proceeding  from 
left  to  right  and  from  top  to  bottom. 

The  terminal  assembly  for  bank  cables  which  was  shown  in  more  or 
less  detail  in  Figs.  102  and  103  is  necessarily  compressed  into  that  shown 
in  Fig.  105.  The  sample  chosen  carries  the  two  A  sections  of  one  selector 
board,  with  short-multiple  cable  running  down,  long-multiple  cable  run- 
ning to  another  board,  and  some  of  the  trunk  cables  which  lead  directly 
to  second  selectors  (second  level  to  second  thousand,  third  level  to  third 


120 


A  l  *  TOM  A  TIC  TELEPIION  Y 


thousand,  ninth  level  to  special  selectors,  and  tenth  level  to  long  dis- 
tance). There  may  be  trunks  running  from  other  levels.  If  so,  they 
are  connected  to  the  terminals  on  some  other  board,  which  is  reached  by 
the  long  multiple.  This  sample  is  supports  1,  2,  and  3,  the  topmost 
supports,  which  accounts  for  the  above  mentioned  trunk  cables. 


long 
Mult. 


Znd.Selector 

Znd.Thou.  Znd.Jhou. 


A-Low- 


^iil 


Supports  - 
Levels     I 


->/  2  3 
4        5 


"■% 


6-2 


±± 


-A- High 


\Shorr 


234         56         789         0 
Fig.   105. — Terminal  assembly,  three  supports,  ten  levels,  symbolized  by  private  alone. 


Although  the  sjonbol  of  Fig.  105  seemingly  provides  for  only  one 
set  of  wires  (the  privates,  for  example)  the  same  arrangement  must  be 
understood  as  applying  to  the  line  bank  wires  as  well.  A  full  expansion 
is  shown  in  Fig.  106.  All  the  levels,  except  two  and  three,  are  multipled 
together  everywhere.  Levels  two  and  three  are  cut  off  from  the  short  or 
up  and  down  multiple.     It  means  that  on  each  of  these  two  levels  the 


To  2nd 

Sets,  Etc. 


To  Other 
Boards 


Levels 


Levels 


Pn'va-t-e 
Levels  --->  I  2X45  6  7  8  90 


100  Wires. 

50  Pairs  -■]> 

c       50  Pairs-- 

■S  Private- 

£  \\Even Lev<?l$-7 

stf  z\pdd Levels — I 


Hi 


Mr 

* 

-.. 

: 




8 

,Tt£=-iS» 

iTitrt1  i 

1 

llllllll  MINI 

7 

[ 

TT 

[HIM 

"' 

- 

i::::,  ; , 

mil] 

100  Lines 

'* — Private       r£-C 
l — Even  L  evels  I S-  S> 
r~Odd  Levels^?  * 

<-SO  Pairs       ? 
■<  -SO  Pairs 


Even 


CM 


lb  Supports 
Below 


Fig.   106. — Expansion  of  Fig.  105  showing  separate  terminals. 


two  A  sections  have  one  group  of  ten  trunks  leading  to  second  selectors- 
Part  of  the  ways  in  which  terminals  are  soldered  together  have  been 

reduced  to  definite  symbols,  which  may  be  recognized  wherever  seen. 

Others  occur  not  very  often  or  are  somewhat  complicated  so  that  their 

symbols  are  not  clear  or  recourse  is  had  to  special  notes  and  drawings. 
Those  which  are  most  common  are  shown  in  Fig.  107,  together  with 


TRUNKING,  ITS  PHYSICAL  ARRANGEMENTS  AND  VARIATIONS     121 

an  example  of  one  which  is  too  complicated  to  receive  a  clear  treatment 
without  a  detail  sheet. 

At  A  is  a  terminal  assembly  with  some  of  the  symbols  as  they  appear 
on  the  drawing. 

At  B  are  three  forms  of  the  symbol  for  all  ten  trunks  soldered  through 
on  all  strips. 

C  shows  two  symbols  for  the  attachment  of  a  cable  to  the  top  strip, 
with  all  trunks  connected. 


S 


i  z  i 


Symbol 

and  land  I 


Explanation 
All  Trunks  Soldered 


and  h~H  Cable  from  All  Ten  Trunks,  Top  Strip 


-4—  Bottom  Strip  Hot  Soldered  at  a  II. 

I— — j  Four  Strips,  All  Soldered 
I       I  Bottom  Strip  Soldered  Except  £  or 
More  Trunks  at  Left  Are  Open 

f"1  Two  Upper  Ships,  Soldered.  Two  Lower  Strips  Soldered. 
<~H  Second  and  Third  Strips  Soldered  Excepts  or 
More  Trunks  at  Left  Are  Open. 

B  Four  Strips  Are  Soldered. 
Bottom  Strip  Soldered  Excepts  or 
Less  Trunks  at  Right  Are  Open 
*38  E-38  A-38  E-38   A- 38  E-38 


land 


E|30      Ar30 

1  j® 

B-30 


t   J.   I   I!   I  For  Exact  Number 
C  X  T  ""  T °f  Trunks  Going Into 
.    X.   "S,   j.   X  X  EachCable.See 
I     234-5S7890  Detail  Sheet 
Trunks 
A-36     E-38       Sec+ion"A"arid  Section "E"on  Bay  38 
A-30 


-E-30 


JiJl   z/z/z/z   Z  XFor  Exact  Number 
X   -  —  -  X   X  X  X   X  ±of  Trunks  Going  Into 


B-30 


"  Each  Cable <t See 
Detail  Sheet 


Fig.   107. — Symbols  used  for  terminal  assembly  solderings. 


D  shows  the  bottom  strip  not  soldered  at  all. 

E  shows  all  trunks  soldered  except  on  the  bottom  strip,  where  the 
number  of  trunks  left  open  must  be  indicated  by  figures,  a  foot  note, 
or  a  detail  sheet. 

F  shows  a  few  trunks  left  open  on  an  intermediate  soldering,  in  this 
case  between  the  second  and  third  strips. 

G  indicates  in  general  the  same  as  E,  except  that  G  is  used  if  the 
number  of  trunks  left  open  is  about  one-third  the  total  or  less. 


122 


A  I J  TOM  A  TIC  TELEPHON  Y 


H  shows  the  leading  out  of  cable  from  two  parts  of  a  level  of  ten 
trunks,  five  trunks  going  into  one  cable  and  five  into  another. 

At  /  is  shown  a  case  of  terminal  soldering  which  is  hard  to  reduce  to 
a  simple  symbol.  Part  of  the  trunks  are  left  open  and  there  are  three 
cables  leading  out  from  different  places.  The  best  that  the  symbol  can 
do  is  to  induce  the  installer  to  look  up  the  detail  sheet  for  the  case. 

The  destination  of  a  cable  is  indicated  by  a  letter  and  a  number. 
The  number  is  that  of  the  bay  and  the  letter  is  that  of  the  section  of 
switches  to  whose  jacks  the  trunks  run. 

Trunking  Devices. — The  term  "trunking  devices"  covers  those 
arrangements  and  combinations  of  trunks  which  give  automatic  telephony 
much  of  its  flexibility.  The  trunking  devices  here  discussed  have  been 
tried  out  in  the  furnace  of  experience.     Part  of  them  have  been  originated 


OV6"1 


Tela  1 24 
Telo.Z33 
Telo.3ZI  O-' 
Telo.419  0-> 


HDF 


i-O  Telo.  113 


Line  IDF 


4-Gt-oups  of  Connectors 
100  Group 

'JC  ?0OGroup 

3006roup 

400Graup 


E-g-S 
?  !e c 
it-Si  «s 

«0 


Jo  Lineswi'tches 
Fig.   108. — Party  lines,  line  IDF  and  grouped  connectors. 

by  the  makers  and  part  by  the  users  of  automatic  exchanges.  Usually, 
after  describing  a  device,  the  way  will  be  shown  for  carrying  it  into 
practice. 

Subscriber  Lines  and  Line  IDF. — The  use  of  the  line  IDF  shown  in 
Fig.  91  permits  party  lines  to  be  handled  very  flexibly.  Suppose  that  a 
four-party  line  is  rung  by  four  groups  of  connectors,  each  equipped  to 
ring  with  a  different  frequency.  (Fig.  108.)  Let  the  16-cycle  con- 
nectors be  in  the  group  numbered  "100, "  the  33-cycle  connectors  be  in 
the  group  numbered  "200,"  etc. 

The  party  lines  are  connected  to  the  line  side  of  the  MDF  as  usual. 
They  are  cross-connected  to  the  switchboard  side  so  as  to  reach  the  bank 
contacts  of  the  groups  necessary  to  get  the  frequency  for  ringing  each 
bell  on  the  line.     For  instance,  telephone  124  is  equipped  with  a  16-cycle 


TRUNKING,  ITS  PHYSICAL  ARRANGEMENTS  AND  VARIATIONS     123 

bell.     It  is  cross-connected  to  bank  contact  No.  24  in  the  16-cycle  group 
of  connectors,  numbered  ''100." 

On  the  same  party  line  is  telephone  233.  Hence  the  same  line  is 
cross-connected  on  the  MDF  to  bank  contact  No.  33  in  the  33-cycle  group 
of  connectors,  numbered  "200." 

The  rest  of  the  jumpers  are  connected  according  to  their  needs  as 
clearly  shown.  Lastly,  a  jumper  on  the  line  IDF  connects  one  of  the 
group  of  bank  circuits  of  each  party  line  to  a  lineswitch. 

Suppose  that  subscriber  321  moves  from  his  neighborhood  to  a  region 
where  he  can  easily  be  connected  to  another  party  line.  Suppose  that 
on  a  given  line,  like  one  shown  in  Fig.  108,  the  50-cycle  station  is  missing. 
Then  subscriber  321  can  have  his  telephone  connected  to  this  line  without 
having  his  call  number  changed.  The  MDF  jumper  marked  "x"  will  be 
removed  from  its  old  line  terminal  and  connected  as  indicated  by  the 
dotted  line.  The  dotted  circle  and  line  indicate  the  new  location  of  the 
sub-station.  In  this  way,  although  the  station  is  on  another  line,  it 
will  be  rung  by  the  same  group  of  connectors  as  rung  it  before  the  move. 
This  flexibility  extends  even  to  cases  in  which  all  the  digits  of  the  call 
number  are  different. 

Divided  Bank  Wiring. — Any  100-line  board  which  handles  more  than 
100  telephones  is  almost  certain  to  require  more  than  ten  connectors.  A 
board  whose  chief  load  is  P.B.X.  lines  may  have  as  many  as  27  connectors. 

There  is  also  very  often  need  for  more  than  ten  trunks  to  a  P.B.X. 
This  may  be  supplied  by  using  the  multi-level  group  connector.  Or  it 
may  be  met  by  dividing  the  bank  wiring  of  the  connectors,  so  that  not 
all  of  them  have  access  to  the  same  group  of  trunks  on  a  given  level. 

Suppose  that  level  three  is  set  aside  for  a  certain  P.B.X.,  and  that  we 
have  twenty  connectors  with  the  usual  complete  multiple.  Then  all 
of  the  connectors  will  hunt  over  the  same  ten  trunks  on  that  level. 

If  the  need  arises  for  more  trunks  to  that  P.B.X.,  we  can  divide  the 
multiple  between  the  bank  terminals  of  the  tenth  and  the  eleventh  con- 
nectors. Connectors  "  1 "  to  "  10 ,J  still  have  the  trunks  on  level  3  appear- 
ing at  the  same  place  and  going  to  the  P.B.X.  Connectors  "  11 "  to  "20" 
are  provided  with  ten  new  trunks  which  run  to  the  same  P.B.X.  Now, 
if  a  subscriber  calling  this  P.B.X.  gets  a  connector  in  the  first  group, 
lifting  the  wipers  to  level  3  will  get  him  an  idle  trunk  in  one  group  of  ten. 
But  if  he  gets  a  connector  in  the  other  group,  lifting  the  wipers  to  level  3 
will  get  him  an  idle  trunk  in  the  other  group  of  ten.  Thus  20  trunks  are 
available. 

Heavy  Traffic  from  Keith  Lineswitches. — If  ten  trunks  can  not  handle 
the  traffic  originated  by  100  subscriber  lines,  the  trunking  can  be  increased 
by  splitting  the  lines  into  two  groups  of  50  lines  each.  Each  shelf  of  50 
lineswitches  is  then  given  a  master  switch  and  ten  trunks,  or  seven 
or  eight  trunks  if  that  many  are  sufficient. 


124 


A  UTOMA 11 C  TELEI'HON  Y 


Extreme  cases  have  been  known  in  which  a  reduction  to  sections  of 
25  has  been  used.  But  it  is  believed  that  the  proper  use  of  the  line  IDF  to 
scatter  the  traffic  will  render  this  unnecessary. 

In  every  case,  the  word  "section"  applies  to  a  group  of  switches  which 
are  mounted  adjacent  to  each  other  and  trunk  into  the  same  group. 
In  the  case  of  lineswitches,  the  section  comprises  those  which  are  governed 
by  the  same  master  switch. 


Day 

BUSY    HOUR    CALLS 

Total 
Da  i  \y 
Calls 

Connector  Switches 
1         2        34-5G         789        10      Total 

20 

59 

45 

34 

27 

13 

5 

4 

4 

0 

0 

191 

1274 

21 

55 

37 

60 

37 

19 

6 

1 

0 

0 

0 

215 

1229 

22 

66 

68 

63 

40 

23 

15 

5 

0 

0 

0 

280 

1536 

23 

6S 

62 

57 

42 

28 

9 

3 

0 

0 

0 

269 

14  53 

24 

57 

48 

48 

32 

15 

3 

0 

0 

0 

0 

203 

1332 

25 

50 

43 

53 

40 

18 

6 

0 

0 

0 

0 

170 

653 

70 


60 


50 

0 

°  40 


=    30 
0 

o 


22 

20 

Aver 
20fo2t 

25 

--'" 

*"•> 

\ 

\ 

V 

~~/ 

V 

\ 

Ave 

\     \  \ 

\  \ 

N 

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\ 

N^ 

S^s 



^"•^ 

20  — 


10 


26.5 

I 
#1 


0.1 


224        21  16  8  3  0.8 

I     Percent  of  Total  Calls  20  to  25 
*2         *3         *4         *5         *6        *7         *8 
Connector  Switch 


0 
*9 


►)0 


Fig.    109. — Telephone  traffic,  private  automatic  exchange  ELS-133. 


Selectors  and  Trunk  Slip. — That  method  of  bank  multipling  known 
as  "bank  slip"  or  "trunk  slip"  was  invented  by  Mr.  A.  E.  Keith  (patent 
No.  831,876).  Its  object  is  to  shorten  the  time  necessary  for  a  selector 
to  find  an  idle  trunk,  to  reduce  the  possibility  of  two  selectors  seizing  the 
same  trunk  and  to  equalize  the  wear  on  the  switches  by  distributing  the 
work. 

If  all  the  selectors  which  use  the  same  ten  trunks  from  a  given  level 
have  their  banks  multipled  point-f  or- point,  all  of  them  will  have  the  same 


TRUNKING,  ITS  PHYSICAL  ARRANGEMENTS  AND  VARIATIONS     125 

trunk  as  their  first  choice.  This  results  in  keeping  the  first  few  trunks 
exceedingly  busy,  and  puts  very  little  work  on  the  last  ones.  This  is 
shown  by  the  traffic  record  of  Fig.  109.  The  group  of  trunks  and  con- 
nectors whose  traffic  is  shown  was  underloaded. 

If  we  were  to  take  out  of  a  selector  bank  one  level  of  contacts,  we 
would  have  ten  contacts  arranged  on  the  arc  of  a  circle  as  shown  at  "  1," 
Fig.  110.  The  arrow  indicates  the  wiper  which  is  adapted  to  move  from 
left  to  right  over  these  ten  terminals.  The  corresponding  levels  in  nine 
other  switches  are  shown  to  the  right  of  "  1 "  and  are  numbered  in  accord- 
ance with  the  numbers  of  the  selectors  from  which  they  are  taken. 
Notice  that  these  bank  contacts  are  multipled  together  with  the 
"straight"  multipling,  trunk  No.  1  being  attached  to  contact  No.  1  of 
each  bank,  and  the  others  to  their  respective  contacts,  point  for  point. 
By  this  method,  each  switch  has  the  same  trunk  as  its  first  choice. 

The  advancing  bank  slip  is  shown  in  Fig.  111.  Bank  No.  1  is  wired 
straight,  that  is,  trunk  1  to  contact  1,  trunk  2  to  contact  2,  etc.  On  bank 
2  all  the  trunks  are  in  the  same  order,  but  have  been  advanced  one  step. 
Trunk  1  is  on  contact  2,  trunk  2  is  on  contact  3,  etc.  It  will  be  noted 
that  switch  1  has  trunk  1  as  its  first  choice,  switch  2  has  trunk  10  as  its 
first  choice,  switch  3  has  trunk  9,  switch  4  has  trunk  8,  etc.  This  method 
has  never  been  used  in  practice,  within  the  knowledge  of  the  writers. 

The  retrograde  bank  slip,  Fig.  112,  is  the  one  which  is  in  general  use  in 
most  automatic  exchanges  As  in  the  two  other  multiples,  the  first 
bank  is  wired  straight.  Bank  2  is  wired  with  the  trunks  in  the  same 
order,  but  slipped  one  step  back.  Trunk  1  is  on  contact  10,  trunk  2  is 
on  contact  1,  trunk  3  is  on  contact  2,  etc.  By  this  means,  the  first 
choice  of  each  switch  is  made  to  be  the  trunk  whose  number  corresponds 
with  the  number  of  the  switch  on  the  shelf.  Switch  5  has  trunk  5  as  its 
first  choice,  switch  8  has  trunk  8,  etc.  This  system  has  the  advantage 
of  simplicity  and  ease  in  tracing  calls. 

The  rule  for  tracing  a  call  forward  (from  a  first  selector  to  a  second 
selector,  etc.)  is  to  add  to  the  number  of  the  switch  the  number  of  the 
bank  contact  upon  which  the  wipers  are  resting,  and  subtract  one.  Thus, 
if  switch  No.  3  be  resting  on  contacts  5  of  its  own  bank,  it  will  be 
connected  to  trunk  7,  for  3  +  5—1  =  7.  Examination  of  Fig.  112  will 
show  the  correctness  of  the  calculation.  If  switch  No.  9  be  resting  on 
contacts  No.  5  of  its  bank,  we  apply  the  rule  thus,  9  +  5  —  1  =  13. 
There  being  no  trunk  No.  13,  we  subtract  10,  and  find  that  the  wipers  of 
the  switch  in  question  are  resting  on  trunk  No.  3. 

There  are  two  places  at  which  the  trunk  slip  may  be  applied,  in  the 
bank  wires  (from  bank  to  bank),  or  in  the  multiple  cable  which  connects 
the  different  selector  shelves  together.  The  former  is  illustrated  in  Fig. 
113.  Let  the  four  single  selector  levels,  1,  2,  3  and  4  at  the  top  represent 
any  given  level  in  the  ten  selectors  on  shelf  A  in  the  first  thousand  (1000). 


AUTOMATIC  TELEPHONY 


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TRUNKING,  ITS  PHYSICAL  ARRANGEMENTS  AND  VARIATIONS     127 


{{W  >)))) 


CUTTT777J 


lllMP4j 


m 


WM)){[im))\[mw 


Multiple 
Cable  ■  i 


\,    N*    \    \< 


■Bonk  Cab/e 


u. 


Rack 
A-  1000 


Rack 
A  -  2000 


Rack 
A- 3000 


To   Other  Shelves 
Fig.   113. — Trunk  slip  in  bank  wiring. 


IJJ 


^iw^Jj^Hi^^^^Wj^viuiij; 


f 


[111  IJUiHIDlJ;  pilUj  P»Sj 


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J  \2 


4     56789    10    I     23 

To  O ther  Shelves 


Fig.   114. — Trunk  slip  in  multiple  cable. 


128 


AUTOMATIC  TELEPHONY 


Below  it  are  another  four  levels  which  similarly  represent  the  ten  switches 
of  the  '  'A  "  shelf  of  the  second  thousand  (2000).  The  third  thousand  is 
also  shown.  At  the  right  of  each  shelf  is  its  connecting  rack,  to  which 
the  contacts  are  wired  strictly  in  order,  as  they  come  from  switch  No.  1, 
contact  No.  1  to  terminal  1,  contact  2  to  terminal  2,  etc.  This  is  true 
of  all  the  shelves. 

Notice  that  the  bank  cable  is  slipped  between  the  adjacent  banks  of 
the  same  shelf,  while  the  multiple  cable  is  run  "straight,"  that  is,  from  a 
terminal  on  one  rack  to  the  corresponding  terminal  on  all  the  other  racks. 
This  causes  switch  No.  1  of  all  the  shelves  to  have  the  same  choice  of 
trunks.  Switches  No.  2  of  all  the  shelves  also  share  their  first  choice 
with  each  other.  In  a  complete  layout  of  ten  shelves  thus  multipled 
together,  we  would  have  ten  switches  whose  first  choice  is  trunk  No.  1, 
ten  whose  first  choice  is  trunk  No.  2,  etc. 

The  method  of  slipping  the  bank  in  the  multiple  cable  is  shown  in 
Fig.  114.  Each  shelf  is  wired  "straight"  or 
"  point  for  point"  to  the  rack.  But  in  con- 
necting up  the  multiple  cable,  a  backward  slip 
is  made,  which  accomplishes  the  same  result 
of  scattering  the  traffic  over  all  the  trunks 
more  evenly.  In  this  case  it  is  the  ten 
switches  of  a  shelf  which  have  the  same  order 
in  the  choice  of  trunks. 

A  very  simple  symbol  for  bank  slip  has 
been  derived  from  Fig.  114.  If  we  examine 
the  terminal  strips  at  the  right  of  the  figure 
we  will  notice  that  the  multipling  from  one 
rack  to  another  exists  in  the  form  of  diagonal 
lines,  each  jogged  over  from  right  to  left  as 
we  proceed  from  the  upper  part  of  the  diagram  to  the  lower.  Each  row 
of  ten  trunks  designated  by  the  word  "rack"  in  Fig.  114  represents  a 
certain  level,  the  same  level  in  each  of  the  shelves.  We  may,  therefore, 
take  these  rack  terminals  and  assemble  them  in  a  diagram  by  themselves. 
This  has  been  done  in  Fig.  115.  The  trunks  are  seen  at  the  top  of  the 
figure,  numbered  from  left  to  right  from  1  to  10.  The  top  row  of  termi- 
nals stands  for  the  certain  level  of  ten  trunks  on  a  certain  shelf  of 
selectors;  the  next  row  below  it  stands  for  the  corresponding  level  in  any 
shelf  of  selectors,  and  so  on,  until  ten  shelves  have  been  connected.  It 
will  be  seen  that  the  retrograde  nature  of  the  bank  slip  is  very  clearly 
shown,  inasmuch  as  each  trunk  slips  backward  one  contact  for  each 
shelf  connected. 

Automatic  exchanges  have  been  installed  with  either  type  of  retro- 
grade bank  slip  as  above  described.  The  slip  in  the  multiple  cable  seems 
to  be  more  in  favor  at  the  present  time. 


Fig.  115. 


-Symbol  for  trunk 
slip. 


TRUNKING,  ITS  PHYSICAL  ARRANGEMENTS  AND  VARIATIONS     129 

Trunk  slip  is  often  applied  by  inserting  in  the  terminal  assembly  a 
pair  of  "jumpered  terminal  strips."  These  two  strips  are  wired  together 
by  jumpers  which  lie  in  the  assembly.  If  a  slip  of  one  is  desired,  the 
wiring  will  be  like  that  shown  in  Fig.  116.  A  slip  of  five  (Fig.  117)  is 
very  common  because  of  its  usefulness. 


Ill  4**1*90  Straight  End 


Slipped  End 

1234567890       r' 

Fig.   116. — Wiring  of  jumpered 
terminal  strips,  slip  1. 


^k^%^%^5traiyhtEnd 

.  Slipped  End 

U5*5    H890 

Fig.   117. — Wiring  of  jumpered 
terminal  strips,  slip  5. 


The  multiple  cable  between  supports  is  either  straight  or  slipped  in 
the  same  way  as  jumpered  terminals.  Figure  118  shows  the  symbol  and 
the  meaning  for  straight  cable,  and  Fig.  119  for  a  cable  which  is  slipped 
one.  This  is  frequently  called  the  " short-multiple"  cable,  to  distinguish 
it  from  the  "long-multiple"  cable  which  connects  bays  together. 


I  l  2, 


Symbol 


Repeat 
for /IN 

Levels 


4     5    &  Meaning 

Fig.   118. —  Multiple  cable  between  supports. 

A  typical  case  of  trunk  slip  is  worked  out  in  Fig.  120  showing  how  the 
terminals  are  actually  handled  to  secure  a  given  result.  The  slip  which 
is  desired  is  shown  at  the  top  of  the  figure.  It  is  an  orderly  slip  from 
section  to  section  in  alphabetical  order.  But  the  sections  are  grouped 
by  twos  on  the  supporting  shelves.  Sections  A  and  F  occupy  the  same 
shelf,  B  and  G  are  together,  etc. 


SL.l 


Symbol 


4    5  6  Meaning 

Fig.   119. — Multiple  cable  between  supports,  slipped  1. 

The  terminal  assembly  to  secure  this  slip  is  shown  in  the  middle 
of  the  figure.  The  departing  long-multiple  cables  are  attached  to  the 
top  strip  of  supports  1,  2,  and  3.  The  next  strip  carries  the  bank  cable 
rom  section  A.  The  next  two  are  jumpered  terminals  with  a  slip  of 
five.  The  free  ends  of  these  upper  three  strips  are  bent  and  soldered 
together  as  was  shown  in  Fig.  103 .  This  carries  the  banks  of  section  A 
straight  out  to  the  trunks. 

9 


130 


AUTOMATIC  TELEPHONY 


The  banks  of  section  F  come  in  on  the  strip  which  is  next  to  the  bot- 
tom. The  bottom  strip  carries  the  short-multiple  cable.  These  lower 
three  strips  are  soldered  together. 

Supports  4-5-6  have  a  similar  arrangement  beginning  at  the  top. 
Sections  B  and  G  are  brought  together  with  a  slip  of  five.     But  below 

To  Second  Selectors 
\      Z34-567      8     9     10 


Slip  Desired 


Supports 

Slih-5- 


hkkk 


kkkk 


4-5-6 


7-8-9 


10-11-12 


SLrS^Z 


SL-5~>2 


I>-|4-I5       SLtSM 


Terminal  Assembly  to  Secure  Desired  Slip 


Short  Mult, 


J^mp  Term 
SLS 


i    I    i   i   i    i   l   i 


Scheme  of  Means  Used 
Fig.   120.- — Typical  case  oi  trunk  slip  worked  out. 

these  four  strips  lies  the  strip  which  carries  the  other  end  of  the  short- 
multiple  cable  mentioned  above.  The  latter  has  a  slip  of  one.  There- 
fore, section  G  is  connected  to  F  with  a  slip  of  one,  indicated  in  another 
way  by  the  small  sketch  "scheme  of  means  used"  found  at  the  bottom 
of  the  figure.     Since  A  is  slipped  five  from  F  and  B  is  slipped  five  from 


TRUNKING,  ITS  PHYSICAL  ARRANGEMENTS  AND  VARIATIONS     131 

G,  and  in  the  same  direction,  A  and  B  have  a  slip  of  only  one  with  regard 
to  each  other.  This  is  brought  out  graphically  in  the  small  sketch  and 
conforms  to  the  slip  desired  and  shown  at  the  top  of  the  illustration. 

In  the  same  way  the  reader  can  follow  the  slip  secured  for  the  remain- 
ing sections.  By  the  use  of  jumpered  terminals  and  slipped  short- 
multiple  cable,  any  desired  scheme  of  trunk  slip  can  be  worked  out. 

SL-2 
1       Z      3       *    _5_  _§_  f_T_  JL     9      0<- Levels 
SYMBOL  " 


MEANING >25§§S?ePe'a,f  for  All  Levels, 

Fig.   121. — Jumpered  terminals,  slipped  2. 

The  standard  form  of  jumpered  terminals  may  sometimes  be  used  to 
advantage  upside  down.  Figure  121  shows  such  a  form  with  a  slip  of 
two.  Figure  122  shows  the  symbol  and  its  meaning  when  used  upside 
down.     This  secures  advancing  slip. 

Another  trunking  device  which  is  used  is  partial  slip,  that  is,  slipping 
part  of  a  level,  and  wiring  the  rest  of  the  level  straight  or  point-for-point. 

_5L-2 
5YMB0L 


MEANING >J^^^^^^peerf  for/Ill  Levels 

Fig.   122. — Jumpered  terminals,  slipped  2,  upside  down. 

Figure  123  shows  the  symbol  for  this.  The  trunks  which  are  to  be  slipped 
are  indicated  by  a  note,  together  with  the  amount  of  the  slip  (SL-1, 
TRK  1-5,  STRAIGHT  6-10). 

The  conditions  indicated  by  a  symbol  must  be  held  to  apply  to  all 
levels,  unless  especially  noted  to  the  contrary. 

Individual  Trunks. — The  first  automatic  exchanges  had  individual 
trunks  only,  so  that  each  switch  or  group  of  switches  had  but  one  trunk 


■  Each  Level 


I    234-56789   0 

y sp.-i  m.-t-s    T///7"  T  T  T  T  T  W«* 

— *. •mmmHT-6'IO     J 1 11L  11111  This  Way 

Fig.   123. — Symbol  for  partial  slip. 

leading  to  any  other  group.  The  simple  individual  method,  however, 
can  hardly  be  said  to  be  economical  of  trunks,  for  the  reason  that  the 
trunks  of  one  group  might  be  entirely  used  up,  while  idle  trunks  are  still 
available  in  other  groups.  For  this  reason  individual  trunks  if  used  are 
always  combined  with  a  common  group,  through  which  calls  may  be  made 
if  all  the  individual  trunks  have  been  occupied. 


132 


A  UTOMA  TI( '  TELEPHONY 


The  individual  trunk  method  used  by  the  American  Automatic  Tele- 
phone Company  is  shown  in  Fig.  124.  A,  B  and  C  represent  three 
shelves  or  groups  of  first  selectors  and  only  five  trunks  are  shown  out- 
going from  each  shelf.  The  first  trunk  is  individual,  as  from  shelf  A 
a  single  trunk  runs  to  second  selector  a;  from  shelf  B,  an  individual  trunk 
to  second  selector  b,  and  so  forth.     There  are  four  common  trunks  multi- 


First  Selectors 


Second  Selectors  or  Connectors 

a  b  c      i Common 

x\l//    \\l//    \\l//    v\i/,    \\M,    \U//    \\H 

\        \        \       \        \ 


c\      \      \      \ 

Fig.    124. — Individual  trunks  with  common  group. 

pled  to  all  the  shelves,  each  trunk  ending  in  a  connector,  indicated  by 
the  word  "  common." 

The  method  of  individual  trunks  used  by  the  Automatic  Electric 
Company  is  shown  in  Fig.  125.  It  consists  essentially  of  one  or  more 
individual  trunks  and  a  group  of  common  trunks,  the  latter,  however, 

Common    Trunks 
1     23456789/0 


Fig.   125. — Individual  trunks  with  slipped  common  trunks. 

consisting  of  ten  trunks  which  are  given  a  bank  slip  between  the  different 
shelves,  although  only  a  fraction  of  the  total  number  of  common  trunks 
are  used  on  any  one  shelf.  This  method  of  trunking  was  arrived  at  in 
the  following  manner:  Suppose  the  ten  shelves  indicated  in  Fig.  125  to 
have  been  originally  installed  without  individual  trunks  but  with  the 
customary  bank  slip  between  the  shelves.     Then  imagine  the  common 


TRUNKING,  ITS  PHYSICAL  ARRANGEMENTS  AND  VARIATIONS     133 

trunks  to  be  removed  from  the  first  two  contacts  of  each  level  and  to  be 
replaced  by  individual  trunks.  The  result  is  the  condition  of  affairs 
shown  in  Fig.  125.  By  this  means  the  out-trunking  from  100  switches 
(ten  shelves  of  ten  switches  each)  is  increased  from  ten  trunks  to  30 
trunks. 

If  we  consider  any  given  shelf  the  switches  of  which  are  multipled  to 
each  other  point-for-point  (i.e.,  without  bank  slip),  we  will  see  that 
whenever  a  switch  seeks  an  idle  trunk  it  will  stop  on  the  first  one  which 
is  found  disengaged.  This  means  that  the  first  trunk  will  get  the  heaviest 
traffic,  the  second  trunk  less,  and  so  on,  diminishing  to  the  last  trunk 
in  the  level,  which  may  rarely,  if  ever,  receive  a  call.     Tests  have  shown 


Common  Trunks 
2       3       4      5       6      7 


Fig.    126. — Common  trunks  added  to  individual  trunks,  ELS-15. 


that  from  50  to  75  per  cent  of  all  the  calls  handled  by  a  fairly  busy  group 
of  ten  trunks  are  handled  by  the  first  three  trunks. 

From  this  we  may  draw  the  very  natural  conclusion  that  individual 
trunks  offer  a  powerful  remedy  to  relieve  overcrowded  trunk  groups,  so 
that  if  an  exchange  which  has  been  installed  in  the  usual  way  shows 
signs  of  overloading  in  any  particular  trunk  group,  this  may  be  relieved 
by  noting  the  most  busy  shelves  which  feed  into  this  trunk  group  and 
installing  individual  trunks  to  divert  enough  of  the  traffic  to  bring  down 
to  a  safe  figure  the  total  load  remaining  on  common  trunks. 

If  the  exchange  is  laid  out  from  the  start  with  individual  trunks  and 
common  trunks,  they  may  be  arranged  as  shown  in  Fig.  126.  There  are 
fewer  common  trunks  than  found  in  Fig.  125,  but  they  have  much  less 
traffic  to  carry  than  if  they  were  part  of  a  general  group. 


134  AUTOMATIC  TELEPHONY 

Preliminary  Impulse. — It  sometimes  occurs  that  a  single  impulse 
is  sent  in  by  the  telephone  before  the  impulses  which  represent  the  call 
number.  The  cause  of  this  is  not  fully  and  definitely  known,  though 
people  hold  various  ideas  upon  the  subject. 

While  the  receiver  is  being  removed  from  the  hook,  it  is  possible  for 
the  subscriber  to  close  the  circuit  and  open  it  again  momentarily  before 
the  final  closure.  If  the  hook  is  not  properly  shaped,  the  receiver  may 
stick  and  be  a  little  hard  to  remove,  so  that  a  downward  pull  may  result 
after  the  hook  lever  has  reached  the  top  of  its  motion.  Again,  the  re- 
ceiver cord  may  interfere  with  easy  removal  of  the  receiver. 

Calling  devices  now  in  use  require  that  the  subscriber  shall  pull  the 
dial  around  to  the  finger  stop  and  let  it  go,  not  interfering  with  its  return 
motion  in  any  way.  If  the  user  fumbles  at  the  dial,  a  preliminary 
impulse  may  occur. 

The  second  of  these  probable  causes  of  preliminary  impulse  deserves 
only  the  attention  necessary  to  educate  the  public  to  use  the  telephone 

To  Regular 

Second 

Selectors 


Clerk 

Fig.   127. — Special  second  selectors  to  care  for  preliminary  impulse. 

properly.  Improper  use  of  the  dial  is  comparable  to  improper  speaking 
of  the  call  number  to  the  operator  of  a  manual  exchange.  It  is  neces- 
sary and  reasonable  to  ask  people  to  speak  clearly  and  distinctly  to  the 
operator.  It  is  just  as  necessary  and  reasonable  to  expect  people  to 
turn  the  dial  continuously  to  the  stop  and  to  let  it  return  to  normal  un- 
influenced by  the  hand. 

The  former  of  these  probable  causes  is  partly  within  the  province  of 
the  maker  of  the  telephone  instrument,  or  of  the  system  to  which  it 
is  connected.  Attention  may  be  directed  toward  preventing  the  pre- 
liminary impulse,  or  toward  absorbing  it  if  it  occurs.  The  former  calls 
for  such  a  design  of  hook  as  will  make  it  impossible  or  at  least  very 
improbable  for  the  hook  to  descend  enough  to  break  the  circuit  after  it 
has  once  reached  the  limit  of  its  upward  motion. 

The  preliminary  impulse  may  be  absorbed  by  installing  a  few  special 
second  selectors  and  multiplying  their  bank  levels  (except  the  first)  with 
the  banks  of  the  first  selectors.  (Fig.  127.)  If  the  subscriber  dials  any 
digit  from  2  to  0  inclusive,  the  call  will  go  directly  to  the  regular  second 


TRUNKING,  ITS  PHYSICAL  ARRANGEMENTS  AND  VARIATIONS     135 

selectors.  But  if  he  prefaces  the  desired  digit  with  an  undesired  "  1,"  he 
will  get  one  of  the  special  second  selectors.  The  next  pull  of  the  dial  will 
trunk  the  call  to  the  same  level  of  the  first  selectors  as  would  have  been 
obtained  if  the  preliminary  impulse  had  not  occurred. 

This  method  of  treating  preliminary  impulse  apparently  cuts  down 
by  10  per  cent  the  capacity  of  the  exchange.  But  in  practice  much  of 
this  is  recovered  by  assigning  to  the  first  level  the  special  and  miscella- 
neous services  of  the  company.  For  instance,  we  may  arrange  111  for 
trouble  clerk,  1 12  for  information,  100  for  long  distance,  etc.  Line  trouble 
such  as  a  swinging  short  circuit  will  usually  be  thus  automatically 
routed  to  the  trouble  clerk. 

Only  as  many  special  second  selectors  as  are  necessary  for  the  traffic 
need  be  installed. 

Grouping  Sections  of  Selectors. — There  is  great  freedom  in  the 
grouping  of  the  banks  of  sections  of  selectors.  The  mounting  of  two 
sections  on  one  shelf  as  described  previously  makes  it  natural  to  expect 
them  to  be  associated.  But  the  terminal  assembly  permits  other  group- 
ings, for  example : 

1.  A  to  A,  B  to  B,  etc. 

2.  A  to  F,  B  to  G,  etc. 

3.  A  to  B,  C  to  D,  etc. 

By  using  this  liberty,  many  trunking  advantages  may  be  obtained. 

Grouping  Levels  of  Selectors. — Each  level  of  selector  banks  may  be 
grouped  in  such  a  way  as  to  secure  greatest  economy  of  trunks,  with 
little  regard  to  the  grouping  of  the  other  levels.  No  two  levels  need  be 
grouped  alike. 

The  first  level  usually  takes  miscellaneous  services,  preliminary  im- 
pulses, etc.  Its  traffic  is  relatively  light.  Hence,  many  sections  of 
banks  can  have  their  first  levels  in  common.  It  is  not  infrequent  to 
find  all  the  first-selector  banks  in  an  office  multipled  together  on  the 
first  level. 

The  second  level  might  happen  to  lead  to  a  very  busy  office.  If  so, 
the  two  sections  which  share  the  same  supports  may  have  their  own  group 
of  ten  trunks.  If  this  is  not  enough,  each  section  may  have  its  own  group 
separate  from  all  others. 

There  are  two  general  methods  of  arranging  the  trunks  from  first- 
selector  banks  to  second  selectors:  (a)  treat  the  second  selectors  as  if 
they  were  in  one  group,  (b)  treat  them  as  if  in  separate  groups.  The  for- 
mer is  more  economical  of  trunks  and  second  selectors,  and  requires 
approximately  the  arithmetical  mean  between  the  number  required  for 
a  true  single  group  and  the  number  required  for  separate  groups  of  ten 
trunks  each. 

In  applying  method  A,  the  number  of  second  selectors  is  first  ascer- 
tained, enough  to  carry  the  total  traffic  from  the  first-selector  banks  under 


136 


A  UTOMA  TI( '  TELEPHON  Y 


the  conditions  indicated  above.  Then  scatter  the  trunks  from  the  first- 
selector  bank  sections  over  these  second  selectors  so  as  to  divide  the  load 
as  evenly  as  possible.  The  first  and  second  choice  of  each  first-selector 
section  should  fall  on  different  second  selectors  from  those  of  other  sec- 
tions, as  far  as  possible. 

In  applying  method  B,  multiple  onto  each  section  of  (ten)  second 
selectors  as  many  sections  of  first  selectors  as  will  secure  the  proper 
traffic  load.  Provide  as  many  such  sections  of  second  selectors  as  are 
necessary  to  care  for  all  the  first  selectors.  There  will  thus  be  a  number 
of  groups  of  trunks  (ten  each)  which  are  independent  of  each  other. 
Usually  there  will  be  a  last  group  which  is  very  lightly  loaded. 


First  Selectors(OneLevel'Onlt)) 
AG  BH  CI  DJ  E    K 

AG  B   H  C    1  D  J  E 


I  High  Bay 
F    L  Low  Bay 


Banks, 


» —  l — i  —  1  —  i  —  i  —  i  —  1 — 1  —  i  —  i  - 

Z-Z-l-Z-Z-l-l-l—  l-l-l-z- 
5—3-3-3—3-3-3—3-3-3-3-3—- 

4-4—4—4—4—4-4—4—4-4—4—4- 
5— 5-5-5-  5-  5-5-  5-5-5-5-5-5-5  -5-5-  5  -5-  5-5-  5-5-  5-5- 
6—6-6—6  —  6-6-6-6- 6— 6— 6-6-6-6— 6 -6- 6-6-6- G- 6 -6-6-6 - 
7-7-7-7-7-7-7-7-7-7-1-1-7-1-7-7-7  -7  -7-  7-  7  -7-  7-7  - 

8-8-8-8-8-8-8-8-8-6-8-8-8-8-8-8-8-8-8-8-8-8-8-8- 
9- 9-9- 9 -9- 9 -9- 9-9 -9-9 -9 -9 -9- 9-9- 9 -9 -9-9- 9 -9- 9_9_ 
0—  0-0-0  —  0—0-0  —  o-o-o-  0-0-  0-0-  0-0-  0-0-  0-0-0  - 0-O-O - 


4_X  Second 
5_^S  Selectors 

v-JTo3nl.Sel. 


Number  of 
Bank  Contacts 


..->i—  I  —1—  1  - 1  - 1  —  j—  1—  1  -  1-  1  -  1- 

Z-l-l-l-  l-Z-Z-Z-  i-l-Z-i- 

3-3-3 -3- 3 -J- 3- 3-3 -3- 3-3- 
4 -4—4 -4-4 -4-4-4 -4-4-4-4- 


From  [. 
Other  }  - 
First  \- 
Selector^  - 


-4-0  Second 
■s-*)  Selectors 

-s-+)To3rd.S?l. 


Fig.   128. — Selectors  trunking  into  14  switches  (scheme). 

Method  A,  the  scattering  of  groups  of  trunks  over  other  groups  of 
trunks  to  save  apparatus  has  been  very  greatly  used  in  the  past.  A 
description  of  some  of  the  arrangements  will  show  what  is  available  to 
the  engineer. 

All  the  selectors  in  a  selector  board  of  240  switches  may  be  trunked 
(on  a  given  level)  to  one  and  a  fraction  sections  of  the  next  selector  ahead. 
(Fig.  128.)  Each  of  the  first  selectors  is  indicated  by  a  point  and  semi- 
circle, the  latter  is  the  bank. 

Take  section  A  high  as  an  example.  Its  ten  trunks  are  attached  only 
to  the  ten  second  selectors  shown  at  the  right.  The  small  numbers  at 
the  intersections  of  the  lines  indicate  the  number  of  the  bank  contact  on 
the  first  selectors.     Thus,  contact  1  goes  to  second  selector  No.  1,  contact 


TRUNKING,  ITS  PHYSICAL  ARRANGEMENTS  AND  VARIATIONS     137 


2  goes  to  second  selector  No.  2  etc.  Half  the  first  selectors  are  multipled 
alike. 

The  second  half  of  first  selectors  (sections  DJ  to  FL)  have  separate 
second  selectors  for  their  first  four  contacts,  but  share  the  last  six  with 
the  other  first  selectors.  Since  most  of  the  traffic  takes  the  first  few 
contacts  of  a  level,  this  arrangement  permits  the  last  six  second  selectors  to 
carry  all  that  spills  over. 

Figure  129  shows  how  such  a  scheme  is  worked  out  in  practise.  It 
will  be  seen  that  it  is  possible,  by  properly  resoldering  the  terminals  on 


Second 
Selec+ors 
I    I    2>     4 


Second  Selectors 
I     134567890 


First 
High    Selector 

^y  Aii3anks 
A  G 


-  Short  Multiple 
Cable 


Contacts  on  one level    1    H  45   &  1  8  9  0 
Fig.    129. — Selectors  trunking  into  14  switches  (terminal  connections). 

the  support  for  D-J  high,  to  let  the  second  half  of  the  first  selectors  use 
the  second  section  of  second  selectors  alone,  freeing  the  first  section  for 
the  sole  use  of  the  first  half  of  first  selectors. 

Another  method  where  the  traffic  is  light  is  to  multiple  together  all 
the  sections  of  a  board  (two  bays)  and  treat  them  as  one  section,  then  to 
combine  several  such  units  so  as  to  deliver  their  traffic  into  fewer  sections 
than  there  are  units.  (See  Fig.  130.)  Here  three  boards,  two  bays 
each,  are  trunked  into  two  sections  of  second  selectors.  Each  has  its 
own  first  five  contacts  individual.  Bays  1-2  have  1  to  5  of  the  A  section 
of  second  selectors.  Bays  3-4  have  1  to  5  of  the  B  section.  Bays  5-6 
have  6  to  0  of  the  B  section.     All  are  common  on  6  to  0  of  the  A  section. 


138 


A  UTOMA  TIC  TELEPHON  Y 


In  another  case,  all  the  A  sections  of  selectors  in  a  given  number  of 
frames  are  multipled  together  with  trunk  slip.  (See  Fig.  131.)  We 
now  dispense  with  the  symbols  for  selectors  and  use  letters  and  figures  to 


First  Selectors  (One  Level  Only) 
Bays  Numbered 


dank  ._>' 
Contact'  j 
Numbers,    A 


6 

7 

8 

9 

) 


dank  _.-•"'  2" 

Contact   j. 

Numbers  4  . 

5- 


5^<2nd.Selectors 

0^)  ToSni.Scl. 

> 

■  l-S\ 

-3-0 

-5-5   B 
-6-0 

■7-5 


7c  J/-^/.  5<?/. 


Fig.   130. — First  selector  trunking,  3  boards  to  2  groups  of  trunks. 

indicate  sections.     The  G  sections  are  likewise  multipled,  but  on  another 
group  of  ten  trunks.     A  third  group  of  selector  banks,  the  E  sections, 

Selector  Banks  (One  Level  Only) 

SectionA  AAAAAAAGGGGGGGGEEEEE    EEE 
Bay    12345678    1234    5   G    7    8    I    2  3   4    5    6  1    & 


1-0-9-8-1-6-5-4- 
j_i_o-9-8-i-&-5- 
3-2-1-0-9-8-1-6- 
4—3—2-1-0-9—8-7- 
5— *— 3-t-i-o— 9-8- 
6-5-4-3-2-1-0-9- 
7— 6-5-4-J-Z-I-0- 
8-7-6-5-4-3-2-'- 
9-8-7-6-5-4-3-2- 
0-9-8-7-6-5-4-3- 


-1-0-9-8-7-6-5-4- 
-2-1-0-9-8-7-6-5- 
-3-2- I-0-9-S-7-6- 
-4-3-2-1 -0-9-8-1- 
-5-4-3-2-1-0-9-8- 


1-0- 

2-'- 

3-2- 
4-3- 
5-4- 
6-5- 
7  -6- 
8-7- 


9-8-1-6-5-4 
0-9-8-1-6-5 
1-0-9-8-1—6 
2-1-0-9-8-1 
3-2-1-0-9-8 
4-3-2- 1-0-9 
5-4-3-Z-l-O 


-0-9-8-7-6-5-4-1 


5-4-3  —  2-1-0-9- 
5-4-3-2-1-1-6-5-4-3-2-1-0- 
6-5-4-3-J-8-7-6-5  — 4-3-2- i- 
7 -6_&_  4- j_ 9-8-1-6- 5-4- 3-2- 


!  Selectors 
A- 53 


G-53 


Fig.   131. — Selector  trunking  (one  level)  3  groups  of  selectors  to  2  groups  of  trunks. 


are  now  divided,  half  of  the  trunks  going  to  the  A -53  section  and  half  to 
the  G-53  section.  Because  each  kind  of  selector  bank  (A,  G,  and  E)  is 
slipped,  the  traffic  is  very  effectually  scattered  over  the  20  trunks. 


TRUNKING,  ITS  PHYSICAL  ARRANGEMENTS  AND  VARIATIONS     139 


Two  groups  of  selectors  may  share  15  trunks.  (See  Fig.  132.)  Bays 
1-2  and  3-4  have  individual  trunks  for  the  first  five  contacts,  but  common 
trunks  for  the  last  five.  In  this  way  four  groups  of  selectors  use  three 
groups  of  trunks,  ten  trunks  each. 

Toll  selectors  are  sometimes  arranged  as  shown  in  Fig.  133.  Each 
section  of  selectors  has  two  individual  trunks.  The  remaining  eight 
contacts  are  common  to  two  sections  alone.  Groups  A-23,  G-23  etc. 
contain  individual  trunks  and  common  trunks,  but  group  D-23  contains 
the  individual  trunks  for  sections  G,  H,  I,  J,  and  K.     If  the  switches  of  a 

Selec+or  Banks.  (One  Level  Only) 

Bays  Numbered 
)-,Z  3-4  5-6 


I  - 
2- 
3- 

4- 
*5- 


i 


Bank       _--->3 

Contact 

Numbers 


■Banks 


A-  Z 


A- 3. 


Fig.   132. — Selector  trunking  (one  level)  2  groups  of  selectors  to  15  trunks. 

section  have  their  adjacent  banks  slipped,  then  the  traffic  is  all  the  better 
distributed,  but  the  individual  trunks  will  occupy  different  positions  on 
different  switches. 

Selector  banks  are  often  grouped  as  if  related  in  varying  degrees. 
(See  Fig.  134.)  In  this  case  each  section  has  three  individual  trunks, 
three  sections  have  five  trunks  in  common,  and  six  sections  have  the  last 
two  contacts  common.  This  arrangement  permits  one  group  of  ten 
trunks  (A -37)  to  care  for  the  first  four  contacts  of  three  sections  (A 
high,  G  high  and  A  low).  Also  one  group  of  ten  trunks  (E-37)  cares  for 
the  last  six  contacts  of  six  sections  (A  and  G  high,  A  and  G  low,  and  B 
and  H  high).     In  the  same  way  the  other  six  section  sare  handled  by 


140 


AUTOMATIC  TELEPHONY 


groups  C-37,  D-37  and  F-37.     To  a  certain  extent  this  equalizes  the 
load  on  the  sections  to  which  the  traffic  goes. 

Another  arrangement  (Fig.  135)  gives  each  section  of  selectors  the 
first  four  contacts  as  individual,  makes  two  sections  common  on  the  next 
two  contacts,  three  sections  common  on  the  two  following  contacts,  and 
12  sections  common  on  the  last  two  contacts  of  the  bank.  Thus  sections 
A  and  G  high  occupy  all  of  the  ten  trunks  of  group  (7-29,  using  trunks  5 

Selector  Banks  (One  Level  Only) 
AGBHCIDJ         EK 


t=t 


■  =  V 


-A-ZS 


-6-25 


B-23 


'H-23 


-C-23 


-D-23 


Fig.   133. — Selectors  trunking  (one  level).     Sections  grouped  by  twos. 

and  6  for  the  common  two  on  contacts  5  and  6.  Sections  A  and  G  high 
and  A  low  have  contacts  7  and  8  common  on  trunks  1  and  2  of  group 
vl-35.  All  the  sections  use  trunks  3  and  4  of  group  A-Z5  for  their  ninth 
and  tenth  contacts. 

Those  selectors  which  trunk  to  connectors  work  under  slightly  differ- 
ent conditions  from  other  selectors,  because  the  connectors  are  not 
limited  to  sections  of  ten.  As  many  as  27  connectors  have  been  mounted 
on  one  lines  witch-board,  all  their  banks  being  common.     Since  there  are 


TRUNKING,  ITS  PHYSICAL  ARRANGEMENTS  AND  VARIATIONS     141 

many  five  digit  exchanges,  it  has  become  common  to  speak  of  these 
selectors  as  "third  selectors,"  although  in  smaller  and  in  larger  exchanges 
they  are  not  thirds. 

Fourteen  connectors  may  receive  traffic  from  selectors  as  shown  in 
Fig.  136.  Eight  sections  of  selectors  scatter  their  traffic  over  the  whole 
number,  except  that  connector  No.  14  is  not  yet  installed.  Connectors 
1  and  2  are  for  toll  service,  although  they  may  be  used  for  local  calls  as 
well.  They  are  called  "combined  toll  and  local  connectors."  The 
toll  third  selectors  reach  only  them.  When  seized  by  a  toll  selector, 
the  combination  connector  gives  the  proper  conditions  for  toll  service. 


High  Say   A       G 
Low  doy  —  \ 


Selsctor  Banks  (One  Level  Only) 
B       H       „        .,       C       I 


Fig     134. — Selector  trunking  (one  level).     Sections  grouped  by  3's  and  6's. 


But  when  seized  by  a  local  selector,  it  acts  like  other  (regular)  connectors. 

The  first  eight  contacts  on  the  selector  banks  are  slipped.  The  last 
two  are  straight.  The  tenth  contact  alone  leads  to  the  toll  connectors, 
so  that  a  toll  connector  will  not  be  taken  for  local  service  until  all  the 
local  connectors  are  busy. 

Figure  137  shows  how  the  terminals  are  arranged  to  secure  this  scatter- 
ing. The  letters  at  the  right  indicate  the  bank  cable  from  the  sections, 
A  and  G  being  on  the  same  supports,  B  and  H  on  another  set,  etc. 

Ten  of  the  connectors  are  brought  onto  the  top  terminal  strip  of  the 
A-G  support.     This  is  all  that  would  be  necessary  if  there  were  only  ten. 


142 


A  U  TOM  A  TIC  TELEPUON  Y 


The  extra  connectors  are  cared  for  by  having  an  extra  pair  of  terminal 
strips  above  the  A-G  assembly,  and  by  inserting  an  extra  strip  between 
the  strips  for  the  pair  of  sections.  Thus  there  is  an  extra  strip  between 
A  and  G,  between  B  and  H,  etc.  There  is  also  an  extra  strip  above  the 
C-F  assembly. 

The  extra  four  connectors  have  their  trunks  wired  to  the  top  extra 
strip.  From  the  companion  strip  (under  it)  run  wires  to  the  extra  strips 
mentioned  above.  The  number  of  each  connector  is  added  to  the  wires 
where  they  terminate  so  as  to  make  it  easier  to  follow  the  trunking. 


High  Bat/  A 
low  Bay 


Selector  Banks  (One  Level  Only) 

G  B       H  CI 

>A       G  B       H  C 


IG-Z9 


A-J5 


Fig.   135. — Selectors  trunking  (one  level)  sections  grouped  by  2's,  3's  and  12's. 


Connector  14  can  be  cut  into  service  by  soldering  down  its  terminal 
at  the  ninth  contact  on  the  C-F  support,  and  unsoldering  the  terminal  of 
the  short  multiple  on  the  B-H  support  above.  This  transfers  the  ninth 
contact  of  the  four  sections  C,  F,  D,  and  J  from  connector  3  to  connector 
14,  leaving  only  four  sections  (A,  G,  B,  and  H)  on  connector  3. 

Individual  trunks  are  provided  for  the  first  contact  of  each  four  sec- 
tions of  selectors.  A,  G,  B,  and  H  have  connector  12,  while  the  other 
four  have  connector  13.     They  are  brought  in  on  the  strips  inserted 


TRUNKING,  ITS  PHYSICAL  ARRANGEMENTS  AND  VARIATIONS     143 

between  the  bank  strips.  In  this  way  the  individual  trunks  can  be 
removed  or  added  at  will,  for  contact  No.  1  has  its  place  in  the  regular 
slipped  eight  trunks.  If  the  individuals  were  done  away  with,  it  would 
(Fig.  136)  throw  each  figure  "  1 "  up  into  the  vacant  space,  which,  because 
of  the  slip,  is  progressively  higher  as  we  go  to  the  right. 

Third  Selector  BanWs  (One  Level  Only) 

Toll 

A    G    B    H  C   I    D  J     sels 

I     I     I     I     I    II     I        i  .  . 

I      |  I    o-o—o-o — I \ -M  Connectors 

0—0— o—o-l  -  I  -  I  -I  — 2 zJ\\Combined  Toll 

9—9  —  9—9-9-9-9-9 5-0  and  Regular 

8— 1-6-5-4- J- i- |  4-0 

7  —6-  5  — 4-3- J-  I  -8 5-0 

6— 5  — 4— 3-f-l  -8-7 4-0 

*— 4-  3-z-l-s—  t-s 7 -o  Reqular  Connectors 

4  —  3-2—1-8-7-6-5 8-0 

J  —  2-  1-8-7-6-5-4 9-Q 

2 —  |  —8  —  7  —  6-5—4-3 10—J 

|— 8  — 7  — 6-5-4-3-Z 11-3 

1—1—1—1—1-1  — |-|  12-0 

i— i-i-  |-i- 1-1-1 u-Q 

I— I — ' — I — r— t — i— i 14-3 

Fig.   136. — Selectors  (3'd)  trunking  into  14  switches   (scheme). 

Seventeen  connectors  may  be  connected  in  two  ways,  of  which  one  is 
shown  in  Fig.  138.  Twelve  sections  of  selectors  are  divided  into  two 
parts,  six  sections  each  for  the  first  seven  contacts.  They  unite  without 
slip  on  the  last  three  contacts. 

Connectors  Combined  Toll 

Regular  •*:  and  Regular 

14    15    12    II     10     9      8     7     6      5     4-      3     Z     I 


Contacts  I    Z  3456769  0- 
Toll  Selector  Bank 


2      3      4      5     6     7      8     9 
Number  of  Contacts 

Fig.  137. — Selectors  (3'd)  trunking  into  14  switches  (terminal  connections). 


The  first  five  contacts  are  slipped  in  the  regular  way,  and  there  are 
no  individual  trunks.     Contacts  6  and  7  are  not  slipped. 

There  are  five  toll  and  regular  connectors,  numbered  1  to  5.  The  toll 
selectors  hunt  over  them  in  that  same  order.     But  the  local  selectors 


144 


A  U  TOM  A  TI C  TELEPIION  Y 


hunt  in  the  opposite  direction,  so  that  what  is  first  choice  for  one  is  last 
choice  for  the  other. 

The  other  way  of  connecting  17  connectors  is  shown  in  Fig.  139.  It 
is  alike  except  that  there  is  more  scattering  on  the  toll  and  regular 
connectors  and  less  on  the  regular  connectors. 

Third  Selector  Banks  (One  Level  Only)     Toll 

3rd 
AGBHCIDJEKFL       Sel. 


0 — 0  —  0  —  0  —  0 — 0  —  0 — 0 0 — 0—0  —  0 

9 — 9  —  9 — 9  —  9 — 9 — 9 — 9  —  9  —  9  —  9 — 9 

8 — 8  —  8 — 8 — 8  —  8- 

7 — 7  —  7—7—7  —  7- 

6  —  6 — 6 — 6 — 6—6- 

5 — 4  —  3  —  2 —  I — 5- 

4 — 3 —  2 —  I — 4—5- 

3  —  2—  I—  5—  4—3- 

Z —  1  —  5  —  4—3  —  2- 

I — 5—4—3 — 2—1- 


Combmed 
Toll  a  Regular 
Connectors, 


I — 5—4  —  3—2- 
/*?■—  1  — 5  —  4—3  —  2 
Numbers  of  dank  Contacts  3  —  2  —  I  —  5 — 4  —  3 
4 — 3  —  2—  1  —  5 — 4 
5 — 4 — 3—2—  I  —  5 
6 — 6  —  6—6—6  —  6 
1  —  7  —  7  —  7  —  7  —  7 

Fig.   138. — Selectors  (3'd)  trunking  into  17  connectors  (scheme  1). 


Third  Selector  Banks  (One  Level  Only)  Toll 
AGBHCIDJEKFL3rd 


o — o— 

9  —  9  — 


.fcfctfcY=l=:J=fc:H 


0 o ' — I — I — ' — ' — ' '* i~ *J    [Combined 

9 9 — 9 — 9 — 9  —  9  —  9—9 3 3— J    [Tolls  Regular 


4 4— } 


Connectors 


7  —  7  — 
6  —  6  — 
5  —  4  — 
4  —  3  — 
3  —  2  — 
2—  I  — 
1  —  5  — 


7  —  7- 
6  —  6- 
3 — 2- 
2 — I- 
I— S- 
6  —  4- 
4 — 3- 


-7  —  7 — 7  —  7 

-6 — 6 — 6 6 

-  1—5- 
-5—4- 
-4—3- 
-3  —  2- 
-2—1- 


-7  —  7  —  7- 
-6  —  6—6- 


5- 

-6—3 

-1-0 

-8— J 

-9— v     Regular 
10— J    Conneciors 
1 1-0 


Numbers  of  Bank 


*  9-5 — 4 — 3—2—1  —  5- 

Comets  l-\-)-l-lZV. 

2 —  I — 5—4 — 3—2- 

I  — 5 — 4 — 3—2—  I- 


12- 
13- 
- 14— J 

15- 0 
16— J 

•  n-0 


.■Banks 


Fig.   139. — Selectors  (3'd)  trunking  into  17  connectors  (scheme  2). 


Connector  No.  5  is  not  installed,  but  may  be  added  at  any  time.  The 
arrangement  of  terminals  is  such  that  when  this  occurs,  contact  8  of  the 
second  six  sections  (D  to  L)  will  be  thrown  from  connector  4  to  con- 
nector 5. 

Twenty-seven  connectors  are  arranged  as  shown  in  Fig.  140.  The 
12  sections  of  third  selectors  are  divided  into  three  parts  as  far  as  the 
first  six  contacts  are  concerned.  The  first  five  are  slipped,  the  rest  are 
not.     Each  group  of  four  sections  trunks  to  a  different  group  of  connect- 


TRUNKING,  ITS  PHYSICAL  ARRANGEMENTS  AND  VARIATIONS     145 

ors  for  the  first  six  contacts.  This  scatters  the  local  traffic  over  18 
connectors. 

Contacts  7  and  8  are  common  to  all  and  are  not  slipped. 

There  are  to  be  seven  combined  toll  and  local  connectors,  of  which 
five  are  installed.  On  the  ninth  contact,  the  third  selectors  are  divided 
into  two  parts,  on  the  tenth  contact,  three  parts.  Thus  there  are  only 
four  sections  of  local  selectors  on  the  first  three  connectors,  which  are  the 
first,  second,  and  third  choice  of  the  toll  selectors.  As  shown,  the  fourth 
and  fifth  choice  of  the  toll  selectors  have  six  sections  of  local  selectors 
each  to  share  the  trunk. 


Third  Selector  Banks  (One  Level  Only) 

I       I      I       I     lei 

o — o — o — o — i-i-fyi 


0  —  0 — 0- 


0  —  0 — 0  —  o — \- 

I    I    I    I 

9 — 9 — 9 — 9  —  9 — 9- 


9 — 9  —  9 — 9 — 9 — 9 


-3-3-, 
-4—4-? 
-5-5-": 

G 


ft — S — 8 — 8- 
7 — 7—7 — 7- 
fc— 6 — 6 — 6- 
5—4—3 — 2- 
4  —  3 — Z — I- 
3 — Z — 1 — 5- 
2—1  —  5 — 4- 
1—  5 —  4 — 3- 


Number 
of Bank 
Con  facts    -^ 


-8—8- 
-7—7- 


-8 — 8 — 8 — 8 — 8- 
-7  —  7  —  7  —  7 — 7- 


6  —  6 6 — 6- 

5  —  4 — 3 — Z- 
4—3 — Z — I- 

>3—  Z I — 5- 

Z —  1  — -5 — 4- 
I  — S 4 — 3- 


Comb'med 
Toll  and 
Regular 
Connectors 


-~^  6 —  G — 6 — 6- 

--^  5  —  4 — 3  —  E- 

^--_           4  —  3  —  Z — I- 
"---^3—1—1 5- 

Z—  1—5 — 4- 
I  —  5 — 4—3- 

Fig.    140. — Selectors  (3'd)  trunking  into  27  connectors  (scheme). 


-9-0 

-io-o 
-II- 

-  IE-3 
-13-0 
"  l4"V  Regular 

- 15  -»y  Connectors 

-16-0 

-17-0 

-18- 

-  19 -O 
-2Q-V 
-71- 
-22-0 

23-0 
-Z4- 
-Vi>- 
-Z6-0 
-Z7-0 


danks 


When  the  sixth  and  seventh  connectors  are  put  in,  they  will  be  combin- 
ation switches.  By  changing  the  soldering  of  the  terminal  strips,  sections 
A  to  I  will  be  cut  over  onto  connector  7,  and  sections  D  to  L  will  be 
changed  to  connector  6.  This  leaves  connectors  4  and  5  individual  to  the 
toll  selectors  and  permits  increased  traffic. 

Selector  Section,  Partly  Equipped. — If  there  is  a  section  of  selector 
switches  which  is  very  lightly  loaded  at  the  present  time,  not  all  of  the 
switches  need  be  installed  at  first.  If  seven  or  eight  will  carry  the  load, 
only  that  many  are  installed.  If  desired,  the  trunks  leading  to  the 
vacant  switch  jacks  can  be  made  permanently  busy  by  grounding  the 
release  trunk  of  each.  When  traffic  calls  for  them,  the  additional 
switches  need  only  to  be  hung  on  the  shelf  and  the  ground  connections 
removed. 

10 


146 


A  UTOMA  TIC  TELEPHON  Y 


Since  the  complete  set  of  ten  trunks  is  slipped,  the  busied  trunks  may 
be  the  first  choice  of  some  of  the  switches  which  have  access  to  this  group. 
To  enable  such  switches  to  find  an  idle  trunk  more  quickly,  it  possible  to 
tie  the  jack  of  each  vacant  switch  position  to  some  installed  switch  in  the 
same  section.     (See  Fig.  141.) 

If  possible,  the  switch  to  which  the  trunk  is  tied  should  not  be  the 
first  choice  of  any  of  the  other  sections,  or  at  least  it  should  be  one  of  the 
less  used  switches  of  the  section.  This  device  is  only  temporary  and 
will  be  removed  when  the  section  is  filled  with  switches. 

One  Office  on  More  than  One  Level. — In  a  small  multi-office 
exchange,  it  is  sometimes  advantageous  to  use  several  levels  on  the 
selector  banks  for  one  office.  In  this  case  the  first  selector  is  more  than 
merely  an  office  selector,  it  also  selects  a  group  in  the  desired  office. 
This  has  been  explained  in  the  introduction  to  multi-office  systems, 
page  18.  It  is  a  device  which  is  useful  chiefly  in  small  exchanges, 
although  occasionally  it  may  find  place  for  a  small  office  of  a  large 
exchange. 

From  Banks  of  Selectors 


a 


31 


Vacant 

Fig.  141. 


Switches  Installed 
-Selector  section,  partly  equipped. 


zz 


Combined  Levels  and  Trunks. — Two  levels  on  the  banks  of  first 
selectors  may  be  combined  initially  if  the  traffic  at  the  start  is  light. 
(Fig.  142.)  Suppose  that  it  is  expected  that  districts  3  and  4  will  grow, 
although  at  present  the  traffic  from  the  point  under  consideration  to 
them  is  light  enough  to  be  carried  by  one  group  of  trunks.  In  this  case, 
district  3  would  have  its  office  numbers  assigned  from  one  part  of  the 
banks  of  second  selectors,  and  district  4  from  the  other  part.  (See  upper 
part  of  Fig.  142.) 

If  "  31 "  is  called,  the  first  selector  will  lift  its  wipers  to  the  third  level, 
and  trunk  to  a  second  selector  in  the  switching  office,  and  the  wipers  of 
the  latter  will  be  lifted  to  the  first  level  which  leads  to  office  31.  If  "44" 
is  called,  the  first  selector  will  take  the  fourth  level,  which  is  tied  to  the 
third  level.  This  gives  the  subscriber  the  same  group  of  trunks  as 
before,  but  the  second  selector  will  lift  its  wipers  to  the  fourth  level, 
which  leads  to  office  44. 

As  long  as  the  two  levels  are  thus  combined,  there  can  be  no  office  41  in 
district  4  nor  any  office  34  in  district  3. 


TRUNKING,  ITS  PHYSICAL  ARRANGEMENTS  AND  VARIATIONS     147 

When  either  of  the  districts  grows  so  that  other  offices  must  be  opened 
which  overlap  on  the  numbers  of  the  other,  the  levels  are  separated  and 
new  trunks  run  to  new  second  selectors  for  one  district  (4).  (See  lower 
part  of  Fig.  142.)  This  step  might  be  made  necessary  by  an  unex- 
pected increase  in  the  traffic,  even  if  the  number  of  offices  did  not 
increase  to  the  overlapping  point. 

Not  all  offices  and  districts  need  to  trunk  into  districts  3  and  4  as 
illustrated.  Those  which  are  differently  situated  might  have  separate 
trunks  from  the  beginning,  and  some  might  even  have  direct  trunks. 


i-s 


Combined  Levels  and  Trunks 
for  Initial  Use 


l-S 


Separate  Levels  and  Trunks  \      -^- 
for  Ultimate  Use 


Fig.   142. — Trunking,  inter-office,  two  levels  combined  for  initial  service. 

Combination  use  of  trunks  is  sometimes  practised  if  traffic  for  special 
service  (information,  complaint,  etc.)  is  light,  and  call  numbers  beginning 
with  "l"  are  used  for  that  service  and  recording  toll. 

The  example  chosen  to  illustrate  this  (Fig.  143)  is  an  exchange  which 
is  divided  into  two  parts,  with  one  numbering  scheme  for  the  whole. 
The  center  for  information,  etc.  for  the  upper  half  is  office  No.  3,  while 
office  No.  5  represents  the  conditions  in  any  of  the  other  offices  in  this 
part.  The  center  for  the  same  service  for  the  lower  part  is  office  No .  2, 
and  office  No.  7  represents  the  rest  of  the  offices  in  that  part. 


148 


AUTOMATIC  TELEPHONY 


Special  second  and  third  selectors  are  installed  in  each  office  and 
reached  from  the  first  level  of  first  selectors. 

Toll  is  called  by  "  100. "  This  gives  a  direct  trunk  from  any  office 
to  the  toll  recording  operator. 

Information  is  called  by  "1202."  The  special  second  selectors  have 
their  second  levels  multipled  onto  the  regular  trunks  leading  to  the 
central  office.  In  the  upper  part,  if  any  office  (No.  5)  calls  "12,"  the 
call  goes  to  an  incoming  second  selector  in  office  No.  3.  Calling  "O" 
lifts  the  incoming  second  selector  to  the  tenth  level,  leading  to  special 
third  selectors,  on  whose  banks  are  the  information  trunks,  level  2. 


Office  5 

-Local  Switched. 


Office  3 
lnc.Zrd.5eli.    5pl.3rd.5els,.  ~  ~~1 


[local        

^==r  [Switches   —  —  Complaint  i 
^^^  J  In  formation 


l^  \sMs  =^frnplaJ 
c 1  rzr~ — In  format i 


'T  ■ 


Fig.   143. — Trunking,  recording  toll  and  miscellaneous  service. 


Calling  the  same  number  from  any  office  (No.  7)  in  the  lower  part 
routes  the  call  over  a  regular  trunk  to  office  No.  2,  where  similar  condi- 
tions prevail. 

Complaint  is  called  by  "  1303."  Levels  "2"  and  "3"  of  the  special 
second  selectors  are  tied  together,  so  that  the  result  is  the  same  as  if 
"12"  had  been  called.  This  choice  of  numbers  is  made  to  render  it 
easier  for  the  public  to  remember  them.  The  separation  occurs  on  the 
banks  of  the  special  third  selectors,  because  the  complaint  trunks  are  on 
the  third  level. 

Mixed  System. — The  use  of  the  terms  "thousand-line  exchange" 
"ten  thousand-line  exchange"  etc.,  seems  to  indicate  definite,  fixed 
limits  to  the  growth  of  a  central  office  equipment.     This   is  not  true. 


TRUNKING,  ITS  PHYSICAL  ARRANGEMENTS  AND  VARIATIONS     149 


The  expansion  of  a  system  does  not  depend  upon  its  original  design,  and 
it  may  grow  as  rapidly  or  as  gradually  as  may  be  desired. 

A  thousand-line  exchange  (Fig.  144)  has  originally  not  over  ten  groups 
of  100  lines  each.  To  expand  it  beyond  1000  lines,  insert  second  selectors 
between  the  first  selectors  and  the  connectors  of  one  of  the  lower  hundreds 
(second  hundred,  for  example).     Attach  the  connectors  to  the  same  level 


900 


Connectors 

800  700 


Fig.    144. — Thousand  lines  being  expanded. 

on  the  second  selectors  that  they  formerly  occupied  on  the  first  selectors. 
The  new  numbers  for  these  subscribers  will  be  the  old  numbers  with  the 
first  digit  doubled.  Thus,  245  will  become  2245,  etc.  The  remaining 
levels  of  the  newly  installed  second  selectors  are  now  available  for  open- 
ing new  hundreds,  so  that  the  capacity  is  now  1900  lines  instead  of  1000 
lines. 

Connectors 


l&t.Sels. 

?nd  SelS. 

1               1 

CD 

Fia.   145. — Fourth  selectors  for  party  lines  in  5-digit  system. 

The  next  step  in  growth  is  to  open  a  new  thousand  on  some  other 
level  of  first  selectors,  exactly  as  was  done  on  the  second  level  illustrated. 
In  this  way,  level  after  level  may  be  expanded  until  it  is  a  10,000-line 
system. 

As  the  system  grows,  the  number  of  first  selectors  must  be  increased 
to  take  care  of  the  increased  traffic  caused  by  the  greater  number  of 
telephones. 


150 


AUTOMATIC  TELEPHONY 


Even  if  an  exchange  is  not  being  expanded,  not  all  of  the  subscribers 
need  have  the  same  number  of  digits  in  their  call  numbers.  Party  lines 
are  often  rung  selectively  by  separate  groups  of  connectors,  each  equipped 
with  the  kind  and  arrangement  of  ringing  current  to  which  a  class  of  bells 
will  respond. 

The  selection  of  these  groups  of  connectors  uses  up  the  hundreds  digit 
of  the  call  number.  In  a  5-digit  system,  this  selection  occurs  on  the 
third  selectors.  (Fig.  145.)  This  will  use  up  one  call  number  for  each 
telephone,  the  same  as  individual  lines. 

To  obviate  this  rapid  use  of  numbers,  fourth  selectors  are  installed 
for  the  party  lines  alone.  This  gives  the  party  line  numbers  one  more 
digit  than  the  individual  lines,  6  digits  in  a  5-digit  exchange. 

Midway  Office  Without  Through  Trunking. — Sometimes  between 
two  separate  exchanges  there  is  a  region  which,  because  of  local  condi- 
tions, has  free  service  to  and  from  both  exchanges  although  those  ex- 


Office  No.  4 

Znd.Sels. 
tet-Sels.    Local      Inc 


Office  No. 3 

Bnd.Sels 

tet.Se/s.    local       Inc. 


Fig.   146. — Midway  office  without  through  trunking. 

changes  are  connected  by  toll  service.  It  can  be  arranged  so  that 
although  each  exchange  can  call  the  midway  office,  it  can  not  call  through 
to  the  other  office. 

Assume  the  midway  office  be  No.  2  (Fig.  146)  and  that  the  two 
exchanges  are  represented  by  office  No.  4  and  office  No.  3. 

Office  No.  2  calls  office  No.  3  by  dialing  "3,"  which  secures  a  trunk 
on  the  third  level  of  the  first  selector,  coming  into  the  desired  office  on 
an  incoming  second  selector.  Likewise  the  other  office  is  called  by  the 
midway  office  by  dialing  "4,"  which  gets  a  trunk  from  the  fourth  level 
on  the  first  selector,  leading  to  an  incoming  second  selector  in  office  No.  4. 

Office  No.  3  calls  the  midway  office  by  dialing  "2,"  which  leads  to  an 
incoming  second  selector  in  office  No.  2.  But  there  is  no  way  by  means 
of  which  office  No.  3  can  connect  with  the  trunks  which  lead  from  office 
No.  2  to  office  No.  4,  because  they  do  not  appear  on  the  incoming  second 
selector  banks  to  which  office  No.  3  has  access. 

The  same  conditions  apply  to  office  No.  4  calling  office  No.  2,  and 
trying  to  call  through  to  office  No.  3 — the  latter  can  not  be  done. 


CHAPTER  V 

SUBSCRIBERS'  STATION  EQUIPMENT  FOR  USE  WITH  AUTO- 
MATIC ELECTRIC  COMPANY'S  TWO -WIRE  SYSTEMS 

All  of  the  various  types  of  subscribers'  station  equipment  which  are 
used  in  connection  with  manually  operated  switchboards  are  used  with 
automatic  switchboards  also.  Therefore  a  description  of  subscribers' 
station  apparatus  for  use  with  the  latter  system  must  include  the  following: 

1.  Ordinary  wall  and  desk  telephone  instruments  for  use  at  individual 
line  and  party  line  stations. 

2.  Intercommunicating  systems  of  the  push-button  type  with  and 
without  secret  service  features. 

3.  Manually  operated  private  branch  exchange  switchboards  of  the 
cordless  type. 

4.  Manually  operated  private  branch  exchange  switchboards  using 
cords  and  plugs. 

5.  Full  automatic  private  branch  exchanges  both  with  and  without 
supervised  trunks. 

The  last  of  these,  the  private  automatic  exchange,  has  become  more 
than  merely  sub-station  equipment,  and  is  treated  in  the  chapter  follow- 
ing this  chapter. 

Typical  circuits  and  equipment  used  in  connection  with  the  Automatic 
Electric  Company's  two-wire  systems  for  each  of  these  classes  of  stations 
will  be  taken  up  and  described  in  the  order  in  which  they  have  just  been 
mentioned. 

It  should  not  be  inferred  that  subscribers'  station  apparatus  similar  to 
that  about  to  be  described  is  not  or  may  not  be  used  with  each  of  the 
different  makes  of  automatic  switchboard  equipment,  but  space  in  this 
book  does  not  permit  descriptions  of  the  circuits  and  details  used  for 
adapting  the  various  classes  of  subscribers'  station  equipment  to  each 
of  the  types  of  automatic  systems.  Therefore  this  chapter  is  confined  to 
one  system,  viz.,  the  Automatic  Electric  Company's,  which  is  known  to 
the  largest  number  of  students  of  the  art.  It  should  be  understood  that 
the  purpose  of  the  chapter  is  to  show  fundamental  principles  and  standard 
practices,  which  have  been  adapted  or  may  be  adapted  to  sub-station 
apparatus  connected  to  any  of  the  makes  of  central  office  equipment 
described  in  this  volume. 

First  Class— Subscribers'  Station  using  a  Single  Telephone  or  a  Tele- 
phone with  Extension. — The  photographs,  of  both  a  wall  and  a  desk  tele- 

151 


152 


A  UTOMA  TIC  TELEPIION  Y 


phone  are  shown  in  Chapter  3,  and  with  them  are  shown  the  circuits 
of  each. 

The  circuits  require  the  use  of  direct-current  receivers,  but  ordinary 
polarized  receivers  with  induction  coils  are  also  used  to  a  small  extent 
with  this  system. 


_£^> 


Red  Thread 


n     Orange 
4\Q  J(D  2§  lty\        Thread 

~\   To  Line 


Call  Device 


Fig.    147. — Desk  telephone  wiring  diagram. 


.-£*_ 


£>_ 


rQCni 

Ringer 


^ 


Shunt 
•^Imp. 

Cal/.Device 


r 


3Cond.  Cord. 


^P 


,      Red'    'I     C—i 
!  02  W    Orange 


Call  Device 


Fig.   148. — Wiring  diagram  for  a  wall  telephone  with  an  extension. 


Transmitter 


Shunt 


Fig.   149. — Wiring  diagram  of  a  wall  telephone  with  an  extension. 

Figure  147  shows  the  circuit  of  a  desk  telephone  of  the  direct-current 
receiver  type  just  as  the  instrument  is  wired. 

Figures  148  and  149  show  two  different  arrangements  for  connecting 


SUBSCRIBERS'  STATION  EQUIPMENT 


153 


up  a  wall  telephone  with  an  extension  so  that  when  either  is  called  both 
bells  will  be  rung.  Of  course  if  an  arrangement  is  desired  whereby  the 
bell  will  be  used  at  one  telephone  only,  the  other  bell  is  simply  omitted 
from  the  circuit.  ^ 

Figure  150  shows  an  extension  circuit 
for  a  desk  telephone  of  the  direct-current 
receiver  type. 

When  a  third  wire  is  not  used  between 
a  desk  or  a  wall  telephone  and  its  extension, 
a  biasing  spring  connected  to  one  end  of  the 
ringer  armature  should  be  utilized  to  pre- 
vent the  bell  of  one  of  the  telephones  on 
the  line  tingling  when  a  call  is  made  from 
the  other  telephone.  By  using  this  biasing 
spring  the  tingling  may  be  stopped  although 
it  may  be  necessary  to  reverse  the  connec- 
tions to  the  [ringer  coil  terminals  so  that 
the  discharge  of  the  condenser,  when  calls 
are  made,  will  flow  through  them  in  the 
proper  direction. 

The  calling  device  has  been  applied  to 
several  telephone  circuits.  Among  them 
are  the  "booster"  circuit  of  the  Automatic 
Electric  Company  and  the  regular  common 
battery  circuit  of  the  Western  Electric 
Company.  Other  special  circuits  will  be 
described  in  connection  with  the  apparatus 
with  which  they  are  used. 

The  booster  circuit  (Fig.  151)  employs 
an  induction  coil  to  increase  the  sending 
ability  of  the  transmitter,  while  retaining 
the  non-polarized  receiver.  One  condenser, 
CT,  is  used  for  talking  and  has  2-m.f. 
capacitance.  The  other,  CR,  is  used  for 
the  bell,  and  for  party  lines  using  harmonic 
signalling  has  0.7-m.f.  capacitance.  There 
are  three  shunt  springs,  S,  on  the  calling 
device,  it  being  necessary  to  short  circuit 
the  transmitter  and  the  receiver  separately. 
Dialing  occurs  through  the  3-4  winding  of 
the  induction   coil  without   appreciably   bad  effect. 

The  same  circuit  applied  to  a  wall  telephone  (Fig.  152)  is  simpler  as 
only  two  shunt  springs  are  needed. 

The  Western  Electric  Telephone  with  a  calling  device  (Fig.  153)  is 


154 


A  UTOMA  TIC  TELEPHON  Y 


the  same  for  both  wall  and  desk  sets.  The  same  condenser  is  used  for 
talking  and  ringing.  There  are  three  shunt  springs  on  the  calling  device. 
When  the  shunt  operates,  it  opens  the  receiver  circuit  and  short  circuits 
the  transmitter.  Dialing  is  done  through  the  1-2  winding  of  the  induc- 
tion coil. 

Since  the  circuits  of  telephones  used  with  different  types  of  coin 
collectors  and  meters  are  shown  in  Chapter  IV  they  will  not  be  repeated 
here. 

Second  Class — Intercommunicating  Systems  of  the  Push-button 
Type. — Push-button   intercommunicating  systems   such   as   have   been 

l-i 


Fig.   151.- 


J     4 

-Circuit  of  booster  telephone,  desk. 


widely  used  in  connection  with  manual  central  office  equipment  and 
trunks  are  also  quite  generally  used  at  the  stations  of  subscribers  to 
automatic  telephone  service,  and  are  arranged  with  one  or  more  trunks 
to  the  automatic  central  office.  These  systems  are  convenient  and  well 
adapted  to  private  branch  exchanges  up  to  a  capacity  of  ten  or  15  sta- 
tions, but  larger  sizes  are  so  expensive  to  install  and  so  expensive  to 
enlarge  or  change  that  their  use  is  generally  not  warranted. 

When  but  one  trunk  is  used  for  a  little  system  it  may  terminate  in  a 
lineswitch  and  connector  switch  banks  the  same  as  any  subscribers'  line 


M 


# 


ttt 


5  CD     C 


L-I 


CR 


L-2 


H* 


m 

5    CD 

N 


3        4 

7J5W1T| 


Trownr^- 

2        I 


Fig.  152. — Circuit  booster  telephone, 
wall. 


Fig.    153. — Circuit  of  Western   Electric   tele- 
phone with  calling  device. 


does  at  the  central  office,  but  when  two  trunks  are  used  they  should 
terminate  in  the  banks  of  rotary  connector  switches  and  lineswitches  so 
that  a  subscriber  calling  this  private  branch  will  be  automatically 
switched  to  whichever  trunk  is  idle. 

Incoming  Calls. — An  incoming  call  over  a  trunk  will  ring  the  regular 
telephone  bell.  (Fig.  154.)  When  the  attendant  answers  by  removing 
her  receiver  from  the  switch-hook  and  pressing  the  key  marked  ftTK" 
circuit  is  closed  from  one  end  of  the  signaling  battery  through  the  100- 
ohm  winding  of  the  double-wound  trunk  relay  and  the  key  contact  to  the 
other  terminal  of  the  battery.     The  trunk-line  relay  closes  circuit  from 


SUBSCRIBERS'  STATION  EQUIPMENT 


155 


one  side  of  the  trunk  to  the  other  through  the  500-ohm  holding  coil, 
but  this  circuit  is  kept  open  for  the  time  being  by  the  break  contact  of 
the  key  which  is  open  as  long  as  the  button  is  pushed  down. 

The  trunk  relay  also  closes  circuit  from  one  terminal  of  the  signal 
battery  through  its  250-ohm  winding  through  the  hook  springs  of  the 
attendant's  telephone  to  the  other  terminal  of  the  signal  battery.  This 
circuit  locks  the  relay  until  the  attendant  replaces  her  receiver  on  its 
switch-hook. 

When  the  attendant  responds  to  the  call  she  learns  the  number  which 
the  calling  party  wishes  and  then  presses  the  key  corresponding  to  the 
desired  station.  When  she  does  this,  the  trunk  key  snaps  back  to  normal 
and  closes  the  circuit  already  described  through  the  500-ohm  trunk  hold- 
ing coil. 


To  Automatic 
Public  Exchange  \ 


Fig.  154. — Trunk  circuit  to  attendant's  station  of  a  push-key  inter-communicating  system. 

The  called  person  then  presses  his  trunk  key  and  holds  conversation. 

When  the  attendant  hangs  up  her  receiver,  the  trunk  relay  unlocks, 
leaving  the  called  station  alone  to  hold  the  trunk. 

Talking  current  for  trunk  calls  is  always  supplied  from  the  main 
battery  at  the  central  office. 

Outgoing  Trunk  Calls. — When  a  party  at  one  of  the  stations  of  this 
little  system  wishes  to  make  an  outgoing  call  he  may  do  so  directly  him- 
self by  pressing  either  one  of  the  trunk  keys  in  his  push-button  box  and,  if 
he  finds  that  the  trunk  is  not  in  use,  calling  the  desired  party  by  means  of 
his  calling  device  just  as  if  he  had  a  direct  line  to  the  central  office.  If 
he  wishes  the  attendant  to  secure  the  desired  party  for  him  and  then 
notify  him,  he  signals  and  talks  to  her  as  for  local  calls.  He  then  hangs 
up  his  receiver  and  awaits  notification.  She  connects  her  set  to  an  idle 
trunk  by  using  one  or  the  other  of  the  trunk  keys  in  her  push-button  box 
and  calls  the  desired  party.  She  then  presses  the  key  corresponding  to 
the  line  of  the  local  party  by  whom  the  trunk  connection  was  ordered 
and  thus  signals  the  local  party  and  informs  him  that  his  connection  is 
ready  on  trunk  No.  1  or  trunk  No.  2,  as  the  case  may  be.     He  presses  the 


156  AUTOMATIC  TELEPHONY 

corresponding  trunk  key,  bridging  his  telephone  across  the  trunk,  and 
talks  to  his  party. 

When  the  attendant  presses  the  key  corresponding  to  the  local  party's 
line,  the  trunk  key  used  by  her  to  set  up  the  connection  instantly  snaps 
back  to  normal  position,  but  when  it  does  so  it  closes  the  circuit  through 
the  break-contact  springs  which  are  in  series  with  the  holding  coil  and  the 
springs  of  the  trunk  relay.  The  contacts  of  the  trunk  relay  are  closed 
while  the  attendant's  receiver  is  off  the  switch-hook  after  the  trunk  key 
has  once  been  depressed,  therefore  the  circuit  through  the  holding  coil  is 
closed  and  holds  the  trunk  connection  up  while  the  attendant  talks  to  the 
local  party.  When  the  attendant  replaces  her  receiver  on  the  switch- 
hook  she  breaks  the  circuit  of  the  250-ohm  winding  of  the  line  relay 
which  in  turn  breaks  the  circuit  through  the  holding  coil. 

Secret  Service  Push-button  Intercommunicating  System. — An  ingeni- 
ous type  of  secret  service,  push-button,  intercommunicating  system  for 
use  with  automatic  switchboard  central  offices  is  that  manufactured  by 
the  Corwin  Telephone  Manufacturing  Company.  Like  the  system  just 
described,  the  Corwin  system  is  a  self-operating  exchange  in  which  each 
party  can  establish  all  local  connections  without  the  aid  of  an  operator, 
and  each  party  whose  station  is  equipped  with  a  calling  device  can 
establish  his  own  outgoing  trunk  connections.  One  station,  called  the 
"  attendant's"  station,  is  set  aside  for  answering  incoming  calls  and  trans- 
ferring them  from  the  incoming  trunk  to  the  proper  local  party.  The 
attendant,  or  some  other  party,  whose  telephone  is  equipped  with  a  call- 
ing device  must,  of  course,  make  the  outgoing  calls  for  any  local  party 
who  is  not  furnished  with  a  calling  device. 

Very  extended  descriptions  of  the  circuits  and  mechanisms  employed 
in  this  system  have  been  published  by  its  manufacturers,  and  since  space 
is  not  available  in  this  chapter  to  repeat  the  long  explanation  required 
to  make  the  details  of  construction  and  operation  clear,  it  is  omitted. 

Third  Class — Manually  Operated  Private  Branch  Exchange  Switch- 
boards of  the  Cordless  Type. — As  an  example  of  a  switchboard  of  this 
kind  a  description  will  be  given  of  a  seven-line  three-trunk  board.  The 
equipment  is  mounted  in  a  small  cabinet  (see  Fig.  155)  about  14  in.  long, 
14  in.  high  and  12  in.  deep.  It  may  be  set  on  any  convenient  desk  or 
table.  It  is  practically  self-contained  as  the  only  apparatus  required 
outside  of  the  cabinet  is  a  small  battery  for  operating  the  night-alarm 
buzzer  and  the  operator's  telephone  equipment,  which  consists  of  a 
standard  automatic  desk  telephone. 

The  switchboard  is  designed  to  operate  with  the  usual  battery  and  is 
equipped  with  a  key  to  switch  from  hand  generator  to  ringing  current  fed 
from  the  automatic  central  office.  Each  trunk  and  line  is  multipled 
through  one  of  the  vertical  rows  of  keys.  The  first  three  vertical  rows  of 
keys  on  the  left-hand  side  of  the  switchboard  are  for  trunks  and  the 


SUBSCRIBERS'  STATION  EQUIPMENT 


157 


remaining  seven  are  for  local  lines.  Directly  above  the  keys  are  the 
incoming  signal  drops  for  the  trunks  and  the  visual  signals  for  the  local 
lines.  Above  these  are  located  the  supervisory  signals — three  for  the 
trunks  and  five  for  the  intercommunicating  switching  circuits.  The 
keys  for  the  night  alarm,  generator  switching  and  battery  cut-off  are 
mounted  in  the  lower  rail  of  the  switchboard. 

Functions  of  the  Keys. — The  circuit  of  this  board  is  shown  in  Fig.  156. 
The  throwing  up  of  any  two  key  handles  in  the  upper  horizontal  row  of 
keys  switches  the  keys  together  and  constitutes  switching  circuit  No.  1. 
Throwing  down  the  handles  of  any  two  keys  in  this  row  switches  the 
keys  together  and  constitutes  switching  circuit  No.  2.  Likewise  the  keys 
in  the  middle  row  control  switching  circuits  Nos.  3  and  4.  Throwing  up 
the  handles  of  any  two  keys  in  the  lower  horizontal  row  switches  thorn 


Fig.  155. — Cordless  private  branch  switchboard. 


together  and  makes  switching  circuit  No.  5.  Throwing  down  any  handle 
in  the  lower  horizontal  row  of  local  line  keys  enables  the  operator  to  ring 
that  particular  line.  Throwing  down  either  key  handle  in  the  lower 
horizontal  row  of  trunk  keys  holds  that  particular  trunk. 

Operation. — For  example,  line  No.  5  wishes  to  be  connected  with  line 
No.  2.  The  removal  of  the  receiver  from  the  switch-hook  of  the  telephone 
of  line  No.  5  closes  circuit  which  pulls  up  the  incoming  visual  signal  of  No. 
5  line.  The  operator  responds  to  the  signal  by  throwing  the  answering 
keys  (white)  of  her  telephone  and  line  No.  5  to  connect  the  two  together 
through  any  switching  circuit  which  is  not  busy  at  the  time.  Learning 
that  No.  2  is  the  line  desired  she  throws  the  ringing  key  of  that  line  and, 
if  ringing  current  is  not  furnished  from  the  central  office,  operates  the 
hand  generator.  She  then  throws  the  answering  key  of  line  No.  2  into 
the  switching  circuit  used  to  connect  her  telephone  with  line  No.  5  and 


158 


A  UTOMA  TIC  TELEPHON  Y 


SUBSCRIBERS'  STATION  EQUIPMENT  159 

when  the  connection  is  established  restores  her  answering  key.  When 
the  parties  hang  up  the  supervisory  signal  on  the  switching  circuit  will 
operate  and  indicate  that  the  conversation  is  completed.  The  operator 
then  restores  all  keys  to  normal. 

On  an  incoming  call  from  the  automatic  central  office  the  ringing 
current  sent  out  by  the  connector  switch  operates  the  drop  associated 
with  the  trunk.  The  operator  responds  by  throwing  the  answering 
key  of  her  telephone  and  of  the  trunk  line  into  a  non-busy  switching 
circuit.  When  she  does  this,  the  40-ohm  line  relay,  connected  in  series 
in  one  side  of  the  trunk  and  bridged  by  a  condenser,  operates  and  cuts 
off  the  drop  and  visual  signal  associated  with  the  trunk.  Learning  the 
local  number  wanted  the  operator  throws  the  trunk-holding  key  and 
restores  the  trunk-answering  key.  This  holds  the  trunk  by  bridging  the 
500-ohm  coil  across  it  and  pulls  up  the  trunk  supervisory  or  holding 
signal.  The  operator  is  now  disconnected  from  the  trunk  and  calls  the 
local  number.  When  she  has  the  desired  party  on  the  line  she  throws  the 
answering  key  of  that  line  and  of  the  trunk  on  which  the  calling  party  is 
waiting  into  a  non-busy  switching  circuit  and  restores  the  trunk-holding  key. 

With  this  method  of  operation  it  is  apparent  that  the  operator  can 
speak  to  the  local  party  wanted  and  ascertain  if  he  wishes  to  talk  to  the 
party  who  has  called,  without  the  calling  party  overhearing  her.  If  the  local 
party  does  not  wish  to  talk  to  the  caller  she  can  then  convey  that  infor- 
mation to  the  calling  party,  thus  protecting  users  of  this  little  system 
from  intruders.  Furthermore,  if  the  local  party  wanted  is  not  found  in 
his  customary  place  she  can  endeavor  to  locate  him  by  trying  the  dif- 
ferent stations  on  her  system  and  thus  be  of  material  assistance  to  the 
calling  party. 

When  a  local  party  wishes  to  make  an  outgoing  call  he  signals  the 
local  operator.  She  responds,  takes  his  order  in  the  customary  way, 
restores  her  answering  key  and  connects  her  telephone  through  a  non-busy 
switching  circuit  to  one  of  the  trunks  which  is  idle.  She  then  operates 
her  calling  device  and  secures  the  automatic  party  desired.  The  slow- 
acting  40-ohm  signal  cut-off  relay  of  the  trunk  keeps  the  line  clear  even 
during  the  instants  that  the  circuit  is  opened  by  the  calling  device. 
Meanwhile  the  local  party  may  have  waited  on  the  line  or  have  hung  up 
as  best  suited  his  convenience.  When  she  has  secured  the  desired  auto- 
matic subscriber  she  signals  the  local  party,  if  he  has  not  waited  on  the 
line,  and  throws  the  keys  necessary  to  connect  him  into  the  switching 
circuit  connected  to  the  trunk  on  which  the  automatic  subscriber  is 
waiting.     She  then  restores  her  own  answering  key  to  normal. 

When  the  parties  hang  up,  the  automatic  connection  is  released  and 
the  supervisory  signals  on  the  switching  circuit  and  on  the  trunk  indicate 
the  fact  to  the  operator,  who  then  restores  to  normal  the  keys  used  in 
switching  them  together. 


160 


A UTOMA TIC  TELEPHONY' 


If  desired,  each  telephone  connected  to  a  cordless  private  branch 
switchboard  of  this  type  may  be  equipped  with  an  automatic  calling 
device  so  that  it  will  only  be  necessary,  when  an  outgoing  call  is  to  be 
made,  for  the  operator  to  throw  the  keys  required  to  switch  the  line  of  the 
party  who  desires  to  make  the  call  to  an  idle  trunk  and  he  can  then  proceed 
to  call  whom  he  wishes  without  further  aid  from  her.  When  he  finishes 
she  will  receive  the  usual  supervisory  signal  and  will  restore  the  switching 
keys  to  normal. 


Fig.    157. — Private  branch  exchange  switchboard. 


Fourth  Class — Manually  Operated  Private  Branch  Exchange  Switch- 
boards Using  Cords  and  Plugs. — A  reproduction  of  a  photograph  of  a 
single  position  switchboard  of  this  type  with  a  capacity  of  about  60 
lines  is  given  in  Fig.  157.  This  type  of  board  is  commonly  equipped 
with  regular  cord  circuits  for  interconnecting  local  lines  and  for  connect- 
ing them  to  the  trunks;  and  also  with  two  or  three  toll-cord  circuits 
which  are  used  for  connecting  up  subscribers  for  "through"  calling  and 
long-distance  connections,  when  talking  current  is  to  be  fed  direct  from 


SUBSCRIBERS'  STATION  EQUIPMENT 


161 


162  AUTOMATIC  TELEPHONY 

the  toll  board  or  from  the  switches  at  the  central  office.     This  board  has 
the  customary  supervisory  and  night-alarm  circuits. 

A  general  diagrammatic  scheme  of  the  circuits  is  shown  in  Fig.  158. 
The  line  circuit  is  equipped  with  relay  and  lamp  signals.  The  regular 
cord  circuit  is  of  the  common  battery  type  with  double-lamp  supervision. 
The  supervisory  lamps  operate  through  a  third  conductor  in  the  cord. 

The  toll-cord  circuit  contains  no  relay  except  one  low-wound  super- 
visory relay  connected  in  series  in  one  side  of  the  cord  and  bridged  by  a 
2-m.f.  condenser.  The  trunk  circuit  is  made  two-way  to  economize  in 
the  trunks  required  and  to  simplify  the  operation. 

The  battery  current  for  this  type  of  switchboard  is  generally  fed  from 
storage  batteries  at  the  automatic  central  office.  A  local  battery  of  dry 
cells  giving  24  volts  is  installed  at  the  switchboard  and  multipled  across 
the  pair  of  wires  which  supplies  current  from  the  central  office  storage 
battery.  This  in  connection  with  a  set  of  condensers  of  10  m.f.  bridged 
across  the  battery  leads,  as  shown,  is  sufficient  to  prevent  noises  and  cross 
talk  in  the  private  branch  switchboard  due  to  the  resistance  of  the  cable 
pair  used  to  supply  the  current  from  the  central  office. 

Outgoing  Trunk  Calls. — If  a  calling  party  desires  an  outgoing  trunk 
connection,  the  operator  places  the  calling  plug  of  the  cord  circuit  in  the 
local  jack  of  the  trunk  circuit.  When  she  does  this,  circuit  is  closed 
through  the  jack  springs  from  positive  battery  through  the  high-wound 
bridge  cut-off  relay  B.C.O.R.  winding  to  negative  battery.  This  relay 
opens  the  circuit  through  the  line  relay  L.R.,  which  is  normally  bridged 
across  the  trunk  circuit,  and  closes  the  circuit  of  the  polarized  supervisory 
relay  S.R.  across  the  trunk.  At  the  same  time  that  the  operator  inserts 
the  calling  plug  in  the  out  jack  she  throws  the  calling  device  key  C.D.K.  . 
which  connects  the  calling  device  direct  to  the  trunk  line  and  cuts  off  the 
supervisory  relay  leaving  the  trunk  line  clear  for  the  calling  device.  She 
then  operates  the  calling  device  to  call  the  desired  party  in  the  usual  way 
and,  as  soon  as  she  has  done  so,  restores  the  calling  device  key  to  normal. 
The  connector  switch  used  in  setting  up  the  connection  at  central  office 
automatically  rings  the  called  party.  During  the  operation  of  the  calling 
device  a  guard  lamp  is  lit  through  circuits  clearly  shown  in  the  diagram 
to  warn  the  operator  if  she  does  not  restore  her  key  to  normal  promptly. 

When  the  called  party  responds  the  reversing  battery  type  of  con- 
nector used  reverses  the  direction  of  current  flow  in  its  calling  party's 
loop  and  this  causes  the  polarized  supervisory  relay  S.R.  associated  with 
the  P.B.X.  trunk  circuit  to  operate  and  break  the  circuit  which  was 
established  when  the  plug  was  inserted  in  the  local  jack  through  the 
contact  of  this  relay,  the  sleeve  of  the  jack,  the  sleeve  conductor  of  the 
cord  and  the  supervisory  lamp  of  the  calling  end  of  the  cord. 

If  desired,  any  one  of  the  telephones  at  the  local  stations  may  be 
equipped  with  an  automatic  calling  device  and  the  user  may  ask  the 


SUBSCRIBERS'  STATION  EQUIPMENT  163 

operator  to  connect  him  up  to  a  trunk  so  that  he  can  make  his  own  culls. 
The  operator  can  do  this  by  using  a  toll-cord  circuit  and  by  inserting  the 
calling  plug  of  this  circuit  into  the  toll  jack  of  the  trunk,  which  cuts  off 
all  of  the  supervisory  circuits  of  the  trunk  and  leaves  the  line  entirely 
clear  from  the  local  automatic  telephone  through  to  the  central  office. 

On  such  a  connection  talking  current  is  supplied  to  the  telephone 
from  the  central  office  and  all  supervision  required  is  furnished  by  the 
cord  supervisory  relay.  This  relay  keeps  the  circuit  through  the  super- 
visory lamp  open  until  the  calling  party  restores  his  receiver  to  the  switch- 
hook.  If  the  relay  were  not  slow  acting  it  might  close  the  circuit  through 
the  supervisory  lamp  for  a  fraction  of  a  second  each  time  the  calling 
device  impulse  springs  broke  the  circuit. 

Incoming  Trunk  Calls. — On  an  incoming  trunk  call  the  generator 
current  from  the  connector  switch  used  by  the  calling  party  operates  the 
trunk-line  relay  L.  R.  through  the  2-m.f.  condenser  inserted  in  each  side 
of  the  trunk.  This  relay  locks  itself  mechanically  and  closes  the  circuit 
through  the  call  lamp.  The  operator  responds  by  inserting  the  answering 
plug  of  an  idle  cord  circuit  into  the  local  jack  of  the  trunk.  The  cut-off 
relay  operates,  opening  the  line  relay  and  closing  the  polarized  supervisory 
relay  circuit  as  before  and  mechanically  unlocks  the  line  relay.  The 
operator  then  takes  the  incoming  party's  order  and  completes  the  con- 
nection in  the  usual  way.  The  polarized  supervisory  relay  is  energized 
in  such  a  direction  that  it  operates  during  the  conversation.  When  the 
calling  party  releases,  this  relay's  armature  is  returned  to  normal  position 
by  the  action  of  the  current  through  the  line  relay  of  the  lineswitch 
belonging  to  the  trunk  and  closes  the  circuit  through  the  cord  supervisory 
lamp. 

If  the  incoming  call  should  be  from  the  long-distance  switchboard,  the 
operator  so  soon  as  she  learns  this,  withdraws  the  answering  plug  used  to 
respond  to  the  call  and  inserts  the  answering  plug  of  one  of  the  special  toll- 
cord  circuits  into  the  toll  jack  and  then  uses  the  calling  plug  of  that  cord 
circuit  to  complete  the  connection  to  the  local  line. 


CHAPTER  VI 
PRIVATE  AUTOMATIC  EXCHANGE 

The  private  automatic  exchange  is  radically  different  from  a  manual 
private  branch  exchange.  The  latter  was  developed  out  of  the  single 
instrument  subscriber  station.  More  stations  were  added  as  business 
demanded,  more  subscriber  lines  were  put  in,  and  finally  a  small  switch- 
board was  installed  to  switch  incoming  and  outgoing  calls.  No  matter 
how  large  the  private  branch  exchange  became  it  was  and  is  still  classed 
as  "subscriber  station  apparatus."  It  is  to  that  extent  integral  with 
the  public  telephone  system.  Local  or  interior  traffic  is  incidental — it  is 
imposed  on  the  P.B.X.  because  it  is  there  and  will  serve  after  a  fashion. 
It  was  not  devised  for  local  traffic. 

The  private  automatic  exchange,  on  the  contrary,  was  designed  pri- 
marily for  local  traffic,  within  the  business  concern  which  it  serves. 
Even  for  the  tying  together  of  a  business  by  easy  quick  communication 
it  is  an  efficiency  device  of  high  rank.  But  it  has  proven  to  be  more  than 
this. 

Requirements  of  Modern  Business. — Certain  services  have  become 
necessary  in  the  conduct  of  all  modern  business  organizations.  As 
a  business  grew  from  a  one-man  affair  to  a  great  organization  of  many 
departments  the  first  and  greatest  need  was  a  means  of  audible  inter- 
communication. A  still  larger  growth  and  it  became  desirable  to  provide 
a  paging  system  by  which  any  desired  code  might  be  sounded  simultane- 
ously throughout  all  departments.  As  the  number  of  employes  grew 
and  the  fire  hazards  increased  it  became  necessary  to  provide  a  fire-alarm 
system.  Owing  to  increased  floor  space  and  greatly  enlarged  stocks  of 
valuable  merchandise,  it  was  found  desirable  to  install  a  watchman's 
service. 

To  meet  these  requirements  there  have  been  provided  in  many  in- 
stances a  system  of  push-bottons  and  buzzers,  a  separate  code-calling, 
or  paging  system,  a  distinct  fire-alarm  system  and  an  unrelated  watch- 
man's service,  all  of  this  in  addition  to  an  antiquated  form  of  telephone 
intercommunication.  One  manufacturing  company  had  28  detached 
systems  in  one  building,  and  another  had  over  60  of  them  in  the  whole 
plant.  Though  handicapped  by  being  unrelated  to  each  other,  these 
services  have  proven  to  be  of  great  value. 

Many  establishments  use  all  of  the  services  which  are  applicable  to 

164 


PRIVATE  AUTOMATIC  EXCHANGE  165 

modern  business — some  of  the  services  are  used  by  all  establishments. 
Without  the  private  automatic  exchange  it  is  the  practice  to  use  a"  dis- 
tinct system  to  accomplish  each  of  the  desired  ends.  It  is  much  better 
for  them  to  be  supplied  by  one  system,  in  which  all  the  services  are 
properly  correlated. 

Services  Rendered  by  the  Private  Automatic  Exchange. 

1.  Telephone  system,  with  many  flexibilities. 

2.  Conference  arrangement  by  telephone. 

3.  Code  Call  System. 

4.  Watchman's  Service,  or  Watchman's  System. 

5.  Fire-alarm  System. 

6.  Emergency  Service. 

7.  Credit  System. 

8.  Secret  Listening  or  Detective  Service. 

9.  Dictating  Service. 

10.  Miscellaneous  Services. 

Not  all  of  these  services  need  be  used  or  even  installed  at  first.  The 
P.A.X.  may  be  installed  simply  as  a  telephone  system  without  any 
feature  but  intercommunication.  As  the  business  of  the  establishment 
grows,  the  features  can  be  added  as  rapidly  as  desired. 

1.  TELEPHONE  SYSTEM 

Local  Traffic. — Because  it  is  automatic,  the  P.A.X.  furnishes  tele- 
phone service  continuously,  24  hours  per  day.  It  is  always  available  to 
coordinate  the  activities  of  the  institution.  This  is  as  vital  to  one  or- 
ganization which  exists  for  one  central  purpose,  as  to  the  general  public 
with  its  diversified  objects. 

The  P.A.X.  greatly  increases  the  amount  of  interior  communication. 
Studies  have  been  made  of  traffic  before  and  after  the  installation  of 
P.A.X.'s  and  it  has  been  found  in  enough  cases  so  that  the  statement  may 
be  made  generally,  that  telephone  use  increases  three  to  five  times.  One 
typical  instance  is  found  in  the  offices  of  the  Louisville  &  Nashville 
Railroad,  at  Louisville,  Kentucky,  where  the  total  jumped  from  3000 
calls  per  day  to  10,000  calls  per  day  over  exactly  the  same  number  of 
telephones. 

The  cause  of  the  increased  calling  is  the  greater  ease  of  establishing 
connections  and  of  disconnecting  them. 

In  many  cases  the  interior  traffic  which  is  thrust  upon  a  manual 
P.B.X.  seriously  interferes  with  the  normal  flow  of  calls  to  and  from 
the  public  exchange.  One  publishing  company  found  that  after  their 
private  automatic  exchange  was  installed,  their  incoming  calls  from  the 
public  exchange  increased  15,000  per  year.  Since  no  change  in  their 
organization  or  business  had  occurred  to  account  for  this,  they  believe 


1G0 


A UTOMATIC  TKLKl'UONY 


that  they  wore  formerly  losing  this  number  of  messages  because  their 
P.B.X.  was  clogged  with  interior  traffic.  In  another  case,  that  of  a  large 
stationery  house,  when  the  P.A.X.  was  installed,  the  operator  expected 
to  have  an  easy  time  with  her  job.  But  in  fact,  the  incoming  and  out- 
going calls  passing  through  their  P.B.X.  increased  from  an  average  of 
500  per  day  to  1500  per  day. 

Secretary  Service. — The  past  education  of  important  executives  has 
been  to  discourage  them  from  putting  through  their  own  telephone  calls. 
This  has  been  due  partly  to  the  attitude  of  operating  companies  and 
partly  to  the  inherent  nature  of  manual  telephony.  The  P.B.X.  opera- 
tor, in  many  cases,  became  a  sort  of  secretary  to  the  manager. 

So  deep-rooted  is  this  condition  that  it  has  become  the  standard 
practise  when  installing  a  private  automatic  exchange  to  equip  the  desks 
of  important  officials  with  manual  telephones,  and  arranging  it  so  that 
the  secretary  of  each  can  do  all  the  dialing.     It  has  also  been  observed 


Connector    BCO.t  ,  +  , Presidents  Office 


Secretary 's 
Office 


Banks 

J* 

No.  88   -L 


■I'H" 


■fL 
No-79  -L 


iHT~^ 


to\ 


-fL 


CD  ■   sogZ^4„E 


CutOut 
Key 

Fig.  159. — Secretary  service  (circuit). 


to  be  the  almost  unvarying  sequel,  that  before  long  each  official  demands 
that  his  manual  telephone  be  replaced  with  an  automatic  telephone. 
Thereafter  he  dials  many  calls  himself,  reserving  for  the  secretary  those 
cases  in  which  the  desired  person  is  hard  to  locate,  and  which  constitute 
the  work  legitimately  to  be  delegated  to  another.  Direct  dialing  by  the 
executive  also  mitigates  the  evil  of  telling  the  called  person  to  "wait  a 
minute,"  which  is  the  annoying  accompaniment  of  delegated  calling. 

Secretary  service  is  provided  in  the  P.A.X.  as  shown  in  Fig.  159. 
Assume  that  the  president's  number  is  "88."  The  line  runs  from  the 
switchboard  to  the  president's  office,  through  a  cut-out  key,  to  the 
secretary's  office,  where  the  bell  £-88  is  located  There  are  two  buzzer 
wires  between  the  two  instruments,  operated  by  the  exchange  battery 
without  actuating  the  exchange  apparatus. 

When  a  call  comes  in,  the  bell  5-88  rings.  The  secretary  answers 
the  call.  If  it  is  one  which  the  president  should  answer,  the  secretary 
presses  the  button,  P-l,  which  operates  the  president's  buzzer.  He 
answers  and  the  secretary  gets  off  the  line.     If  the  president  desires  to 


PRIVATE  AUTOMATIC  EXCHANGE  167 

make  sure  of  it.  he  presses  the  cut-out  key,  which  cuts  off  the  secretary's 
telephone. 

When  the  president  desires  the  secretary  to  make  a  call  for  him, 
he  presses  the  button  P-2,  the  secretary  comes  in  on  the  line,  and  the 
instruction  is  given  by  telephone.  The  president  hangs  up  his  receiver 
and  the  secretary  puts  the  call  through,  calling  by  buzzer  when  the  call  is 
ready. 

An  additional  service  is  available  in  the  form  of  a  separate  number 
which  other  officials  may  call  and  get  the  president  directly.  It  is 
done  by  connecting  the  bank  contacts  of  this  separate  number  to  the 
president's  line,  88,  transposing  this  connection,  and  installing  an  addi- 
tional bell  jB-79  on  the  president's  desk.  Because  of  the  transposition  of 
the  lines  and  the  connection  of  each  bell  to  ground  the  ringing  will  be 
selective.  Calls  on  No.  79  will  ring  only  the  president's  bell,  and  he  will 
answer  himself,  because  he  knows  that  the  call  is  coming  from  one  of  a 

Connector  „__ 

Banks     P  J?S$L 


=/        ^       ^ 


Regular  t=/ ^7 ~~7 

"o.Z4     ^        ^        c^^ 


TnnrcrH|i|i[-j|i 

Telo  No.24 


1200*  , 
^       U      ^VWH)'1«H" 

Pick  Up         . 7 -, ^L 

No.  6  6     <=*<=*<=* 

^\       =\ ^ ±A 


Fig.   160. — Double  number  pick-up  (circuit). 

restricted  group  of  officials  who  alone  know  the  number.  Calls  from  all 
others  will  come  in  on  No.  88,  which  ring  the  secretary's  bell  in  the  usual 
way. 

Double  Number  Pick-up. — If  an  executive  who  has  no  secretary  calls 
his  office  from  any  part  of  the  plant  and  finds  the  line  busy  it  is  presumed 
that  the  call  is  for  him.  In  a  manual  system  he  would  flash  the  opera- 
tor and  ask  her  to  transfer  the  call  to  the  telephone  he  was  using.  The 
P.A.X.  gives  the  same  service  as  shown  in  Fig.  160.  Line  24  is  multipled 
throughout  the  various  connector  banks,  and  it  is  this  number  which 
appears  in  the  directory.  The  line  switch  is  now  removed  from  line  86 
(for  instance),  and  the  positive  and  negative  side  of  this  line  are  multipled 
with  line  24. 

The  privates  of  the  two  lines  are  not  multipled,  a  1200-ohm  coil  is 
bridged  from  the  private  of  line  86  to  negative  battery,  should  the 
executive  chance  to  call  his  own  office  when  the  line  is  busy,  he  would 
immediately  release  and  dial  the  "double  number,"  (known  only  to 
himself)  thereby  extending  the  connection  to  his  own  line  even  though  it  is 


108  AUTOMATIC  TELEPHONY 

in  use.  Since  the  privates  of  the  two  lines  are  not  multipled  it  is  quite 
evident  that  the  ground  on  the  private  of  line  24  cannot  cause  the  con- 
nector calling  line  86  to  give  the  busy  tone.  The  connectors  are  of  the 
type  whose  release  is  controlled  by  the  calling  person — the  back-bridge 
relays  can  feed  current  to  each  other  without  preventing  release. 

This  device  is  also  useful  in  case  the  call  to  the  executive's  office  has 
been  answered  by  a  private  secretary.  The  latter  can  call  the  executive 
by  the  code-call  system,  and  he  can  get  in  on  the  call  to  his  office  by  means 
of  the  secret  double  number  pick-up. 

Rotary  Selection  of  Telephones. — The  P.A.X.  connector  incorporates 
the  well-known  group  connector  feature  which  is  used  a  number  of  places 
to  great  advantage.  One  case  especially  is  in  a  concern  where  they  have 
an  elaborate  production  system  which  necessitates  the  workmen  going 
very  frequently  to  pneumatic  tubes  and  sending  reports  of  their  produc- 
tion to  a  dispatcher's  office.  It  is  very  often  necessary  for  the  workman 
to  reach  the  dispatcher  by  telephone  in  order  to  handle  the  system 
properly  and  efficiently.  This  is  taken  care  of  by  a  number  of  tele- 
phones seized  by  rotary  selection  from  a  common  number.  Telephone 
number  one  of  the  group  always  receives  the  first  call,  simultaneous  calls 
being  automatically  passed  on  to  the  other  telephones.  This  results  in 
the  saving  of  a  great  deal  of  time  when  two  or  more  workmen  want  to 
get  the  dispatcher's  office  immediately. 

Loud  Ringing  Devices. — In  many  institutions  a  great  need  is  felt  for 
loud  ringing  devices  to  operate  regularly  with  the  telephone.  To  give 
this  service  a  3-pole  alternating  current  relay  is  wired  in  multiple  with 
the  regular  bell,  or  if  desired,  in  place  of  it.  The  contacts  of  this  relay 
operate  loud-ringing  battery  bells,  lamps,  110-volt  horns  or  bells,  and  in 
some  cases  air  whistles.     Many  combinations  have  been  used. 

Selective  Two-party  Line. — The  P.A.X.  can  furnish  selective  two- 
party  line  service  by  the  simple  scheme  already  shown  in  Fig.  159. 
This  scheme  furnishes  no  ring  back  between  the  two  telephones,  however, 
buzzers  can  be  used.  Although  in  a  great  many  cases  this  divided 
party-line  scheme  is  used  between  telephones  in  the  same  room,  partic- 
ularly where  it  is  desirable  that  the  incoming  calls  be  distributed. 

2.  CONFERENCE  ARRANGEMENT 

To  facilitate  the  close  supervision  required  in  a  large  organization  the 
conference  arrangement  has  been  developed.  This  service  enables 
several  department  heads  or  foreman  to  get  in  on  the  same  line  at  the 
same  time.  This  allows  them  all  of  the  advantages  of  the  conference 
table,  without  requiring  them  to  leave  their  desks. 

To  give  conference  service  some  number,  for  instance  50,  is  designated 
as  the  conference  number,  the  corresponding  line  switch  will  be  removed 


PRIVATE  AUTOMATIC  EXCHANGE 


169 


and  an  impedance  coil  will  be  bridged  across  the  line  contacts.  (See  Fig. 
161.)  In  addition  the  private  multiple  associated  with  line  50  is  opened 
between  each  connector  bank,  and  a  1200-ohm  coil  is  bridged  from  each 
private  contact  to  negative  battery.  We  can  now  call  the  various 
departments  and  instruct  them  to  dial  the  conference  number.  As  each 
department  calls  the  conference  number  his  connector  will  come  to  rest 
with  its  wipers  in  engagement  with  line  50,  following  which  the  ring  will 
be  cut  off  by  the  impedance  coil.  The  number  that  can  get  in  on  the 
conference  is  only  limited  by  the  number  of  connectors. 


soow 

Impedance 


No-50  on  four  Connector  Ban  kz 


Fig.    161. — Conference  connection.     One-hundred-line  P.  A. X.  • 

Two  competing  newspapers  in  Indianapolis,  Indiana,  use  the  con- 
ference feature  of  the  P.A.X.  to  facilitate  the  handling  of  important 
news  from  the  state  capitol. 

From  each  P.A.X.  (Fig.  162)  there  are  lines  and  special  stations  in 
the  press  rooms  on  the  assembly  and  senate  sides  of  the  State  House. 
The  reporters  who  are  detailed  to  state-house  duty  can  call  any  depart- 
ment of  their  paper  by  dialing  two  digits,  and  they  in  turn  can  be  called 
just  as  any  other  station  on  the  newspaper's  P.A.X. 


lOEDITOR  £>*-  — 


|P.A.x] 


L^ 


STATE  CAPITAL 


Diagram  showing  conference  calling  feature  of  .Indianapolis  newspapers'  P.  A.  X.'s. 

Fig.   162. — Conference  calling  feature  of  Indianapolis  newspapers. 

Whenever  anything  of  exceptional  importance  occurs  at  the  Capitol, 
especially  during  legislative  sessions,  the  reporter  of  either  paper  can  get 
his  editor  in  three  or  four  seconds  and  explain  the  circumstances.  If 
the  conditions  warrant  it,  the  city  editor  can  summon  a  telephone  con- 
ference at  once.  He  calls  the  managing  editor,  the  news  editor,  and  the 
press-room  foreman  and  asks  them  to  come  in  on  the  conference  line. 
Here  all  of  them,  including  the  reporter  at  the  Capitol  building,  discuss 
the  matter  and  decide  within  two  or  three  minutes  whether  a  special 
edition  is  advisable.  This  saves  from  15  minutes  to  half  an  hour  in 
determining  what  to  do. 


170  AUTOMATIC  TELEPHONY 


3.  CODE  CALL  SYSTEM 


It  is  of  prime  importance  to  make  it  possible  to  reach  every  man  over 
the  P.A.X.  But  in  many  establishments,  those  most  frequently  con- 
sulted over  the  telephone  find  it  most  necessary  to  be  away  from  their 
desks  most  of  the  time.  To  meet  this  condition  the  code  call  has  been 
developed.  Failing  to  reach  a  man  at  his  desk,  the  person  calling  simply 
disconnects,  dials  the  code  number,  and  immediately  the  man  sought 
hears  the  summons;  going  to  the  nearest  P.A.X.  telephone,  he  dials  the 
answering  number  and  is  connected  direct  with  the  man  wanting  him. 
The  flexibility  of  the  code  call  is  so  great  that  it  can  be  applied  to  one  or 
many  buildings. 

4.  WATCHMAN'S  SERVICE 

The  Watchman's  Service  furnished  through  the  P.A.X.  switches  and 
telephones  is  a  complete  reporting  and  checking  system,  and  in  addition 
to  the  standard  P.A.X.  equipment  requires  only  .a  recording  clock  and 
an  annunciator.  With  it  the  chief  or  supervisor  is  in  constant  touch  with 
all  the  watchmen  wherever  they  may  be.  As  the  watchmen  make  their 
rounds,  they  report  by  momentarily  lifting  the  receiver  of  certain  desig- 
nated telephones.  Their  reports  are  made  visible  (annunciator)  as  well 
as  recorded  (clock),  so  that  the  chief  in  effect  follows  them  on  their  routes 
and  can  reach  any  or  all  of  them  at  any  time  by  either  a  direct  call  over 
the  telephone  or,  in  an  emergency,  by  means  of  the  code  call,  which 
reaches  all  of  them  at  once.  Every  telephone  is  potentially  a  reporting 
station,  consequently  the  daily  routes  can  be  changed  to  suit  the  condi- 
tions of  the  plant.  For  instance,  the  storage  of  valuable  or  dangerous 
material  in  some  unusual  place  for  one  or  two  nights  can  be  covered 
and  reported  on  by  rerouting  the  nearest  man  to  include  the  telephone 
at  the  storage  point. 

When  the  watchman  momentarily  lifts  the  receivers  of  designated 
telephones  while  making  their  rounds,  the  corresponding  annunciator 
drop  falls,  thus  affording  a  check  on  the  location  of  the  watchman. 
At  the  same  time  the  associated  magnet  in  the  clock  operates  to  pierce 
a  hole  in  the  paper  chart  which  is  rotated  by  the  clock.  The  position 
of  this  hole  associates  it  with  the  magnet  operating  and  also  with  the 
time  of  operation,  thus  affording  a  permanent  record  of  the  watchman's 
report.  The  annunciator  cabinet  consists  of  annunciators  which  stay 
operated  until  reset  by  a  push-button  which  operates  all  annunciator? 
in  series,  each  annunciator  having  two  coils,  one  to  operate,  wound  to 
2.r)0-ohms,  and  one  to  reset,  which  is  of  low  resistance  as  it  operates  in 
scries  with  all  the  others. 

At  the  left  of  Fig.  103  will  be  found  three  relays  representing  the  line 
relays    of    three-line    switches.     When   a   given  line   relay  operates  it 


PRIVATE  AUTOMATIC  EXCHANGE 


171 


closes  a  circuit  through  its  associated  pull-down  winding  in  the  usual 
manner,  the  operating  winding  of  an  annunciator  and  a  clock  magnet 
are  in  parallel  with  this  pull-down  winding,  and  will  operate  at  the 
same  time.  The  line  relay  will  close  the  circuit  for  an  instant  only, 
therefore  the  operation  of  the  perforator  will  not  interfere  with  the  rota- 
tion of  the  clock  disk. 

The  supervisor's  office  or  "gate  house"  is  supplied  with  a  regular 
telephone,  number  22  for  instance,  and  the  special  line  number  11. 
This  11  telephone  is  not  equipped  with  a  calling  device,  the  associated 

Annunc/a  tors 
1  Operating 

Resetting 


1 


\ 


PDC 


Line  Relays 

of         t>. 

L'meswitches^ — ^d  / 


\ 


PDC 


CTj~ 


PPC 


+"  '-   battery 


Reset- 


.0    * 

battery 
Fig.    163. — Watchman's  clock  and  annunciator  connections. 

line  switch  is  removed  and  the  private  multiple  opened  between  each 
connector  bank.  Each  private  contact  is  now  connected  through  a 
1200-ohm  coil  to  negative  battery.  From  the  foregoing  it  is  evident  that 
several  persons  (depending  upon  the  number  of  connectors)  can  get  in  on 
line  11  at  the  same  time.  The  use  of  this  device  in  an  emergency  will  be 
discussed  later. 

The  telephone  line  22  (as  shown  in  Fig.  164)  passes  through  a  key  by 
means  of  which  the  telephone  instrument  may  be  removed  from   line 


"#??Z/ 


Telephone- 


Test 


■t-L 


±L  To  Test- 
Connector 


Yale  Lock  ~L 

Fig.    164. — Supervisor's  telephone  circuit. 

22  and  connected  to  a  test  connector.  This  enables  the  supervisory 
watchman  to  cut  in  on  any  line  in  the  plant,  even  though  it  is  busy. 
The  ringer  remains  across  line  22  at  all  times,  therefore  an  incoming 
call  will  be  audiable  even  though  the  supervisor  is  using  the  test  connec- 
tor at  the  time.  Because  of  the  half  M.F.  condenser,  the  bell  rings 
noticeably  weaker  in  one  case  than  in  the  other. 

It  is  quite  probable  that  there  will  be  certain  lines  to  which  the  super- 
visor should  not  have  access.  In  such  cases  lines  will  not  appear  on  the 
banks  of  the  test  connector.     It  is  customary  to  indicate  these  restricted 


172  AUTOMATIC  TELEPHONY 

lines  by  a  certain  designation  on  the  associated  annunciator  panel.  A 
further  restriction  may  be  imposed  by  an  executive  operating  the  Yale 
cut-out  lock,  which  opens  up  the  circuit  to  the  test  connector. 

It  may  be  noted  that  the  clock  is  not  a  watchman's  service,  it  is 
simply  a  check  on  the  supervisor.  The  clock  and  Yale  cut-out  lock  are 
usually  mounted  in  the  office  of  some  executive.  A  dictograph  trans- 
mitter is  usually  placed  in  the  annunciator  cabinet  so  that  the  executive 
can  pick  it  up  from  any  telephone  (the  number  being  known  only  to 
himself)  and  thereby  hear  everything  going  on  in  the  gate  house. 

Many  times  it  is  not  desirable  to  have  a  watchman  enter  a  private 
office  to  report,  in  which  case  the  line  may  be  extended  to  a  push-button 
outside  the  door.  With  certain  lines  are  also  associated  small  open  jacks 
by  means  of  which  the  watchman  can  use  small  pocket  telephones  for 
reporting,  etc.  This  is  especially  desirable  in  some  plants  for  the  use  of 
plain  clothes  men. 

The  watchman's  system  in  combination  with  the  code-call  equipment 
is  very  complete.  The  watchmen  report  by  simply  lifting  the  telephone 
receivers,  pushing  the  buttons  or  inserting  pocket  telephones  into  jacks 
which  throws  up  the  annunciator  in  the  gate  house  and  also  operates 
the  clock.  If  the  watchman  desires  to  talk  with  the  gate  house  (and 
he  is  required  to  do  this  at  certain  intervals)  he  simply  strikes  the  receiver 
hook  twice  connects  him  with  the  11  telephone.  If  the  supervisor  is 
answering  a  party,  the  second  party  who  might  try  to  get  in  at  the  same 
time  will  land  in  on  his  ear  with  a  decided  click.  If  he  has  answered 
some  one  and  restored  his  receiver,  and  they  have  not  hung  up,  the 
second  call  will  ring  his  bell  and  will  also  sound  in  the  receivers  of  all 
parties  in  connection  with  him.  This  makes  a  very  complete  and 
desirable  operation. 

If  the  supervisor  should  want  to  get  all  of  his  watchmen  together,  or 
those  in  a  certain  department,  he  can  do  so  by  means  of  the  code  call,  the 
watchmen  answering  in  on  the  11  telephone.  He  can  see  them  coming 
first  by  his  annunciator,  and  can  immediately  decide  whether  or  not  he 
wants  that  particular  man,  as  the  annunciator  gives  him  the  location  of 
the  calling  watchman.  In  small  plants  that  do  not  have  a  supervising 
watchman  the  code  call  is  invaluable  for  should  there  be  only  two  watch- 
men in  the  whole  plant  they  can  immediately  find  each  other. 

6.  FIRE  ALARM  SYSTEM 

Regular  telephones  have  proven  to  be  in  many  cases  the  best  fire- 
reporting  system,  because  valuable  information  besides  the  mere  location 
can  be  given  to  the  fire  fighters. 

The  code-call  system  gives  as  one  of  its  functions  a  very  excellent 
fire-reporting  alarm.     The  ordinary  fire  box  merely  gives  the  location 


PRIVATE  AUTOMATIC  EXCHANGE 


173 


of  the  person  who  is  reporting  the  fire.  The  fire  itself  may  be  at  a  very- 
different  place.  By  the  code-call  arrangement,  full  information  can  be 
exchanged  between  those  who  are  interested. 

The  fire-alarm  service  consists  of  designating  some  number,  usually 
99,  as  the  fire  alarm.  This  number  is  arranged  the  same  as  the  11  tele- 
phone in  the  gate  house,  except  that  instead  of  having  a  telephone  across 
it,  it  has  a  high-voltage  relay  which  operates  the  general  fire  signal. 
This  signal  should  have  no  connection  whatever  with  the  code-call 
signals. 

When  99  is  called  the  general  fire  alarm  is  sounded,  which  is  a  notice 
that  all  unimportant  telephoning  must  cease  at  once  so  as  to  release  the 
switches  for  certain  officials  whose  duty  it  is  to  come  in  on  99  and  listen 


Fig.   165. — Fire  and  police  box. 

to  the  man  reporting  the  fire  to  the  supervisor.  The  supervisor  came 
in  on  99  by  means  of  his  test  distributer  the  instant  the  alarm 
was  sounded.  The  officials  hear  the  report  of  the  fire  and  in  conference 
decide  what  steps  are  to  be  taken,  and  if  it  is  to  be  kept  a  still  alarm  or 
the  location  given  to  every  one  in  the  plant.  The  supervisor  then  pulls 
the  proper  code  on  his  code-call  equipment  (the  general  fire  alarm  having 
ceased  because  everyone  who  was  in  on  the  conference  has  hung  up)  and 
notifies  everyone  of  the  location  of  the  fire.  On  cards  placed  throughout 
the  departments  are  sketches  of  the  plant,  each  subdivision  bearing  a 
number.  If,  after  the  general  fire  alarm  has  sounded,  the  code  53  is 
rung  on  the  code  call  it  indicates  that'  the  fire  is  on  the  third  floor  of 
building  number  5. 


174  AUTOMATIC  TELEPHONY 

During  the  course  of  the  fire  the  11  telephone  is  always  open  for 
everyone  to  reach  the  supervisor,  the  code  call  to  reach  individual 
officials  and  the  99  number  to  call  all  of  the  principals  together  for  a 
conference.  During  the  course  of  the  fire  the  supervisor  can  pull  other 
signals  on  the  code  call  which  will  notify  everyone  of  the  progress  of  the 
fire  in  another  direction.  He  can  also  use  his  code  call  to  notify  the 
proper  persons  for  arranging  to  clear  certain  sections  of  the  plant. 

Special  Fire  Alarm  and  Police  Box. — A  special  fire  and  police  box 
has  been  developed  in  connection  with  P.A.X.  systems.  In  Fig.  165  is 
shown  two  trip  boxes,  one  marked  "fire"  and  the  other  "police."  As- 
sociated with  this  box  is  a  musolophone  with  a  horn  and  a  dictagraph 
transmitter.  On  breaking  the  glass  in  the  fire-trip  box,  it  automatically 
sets  up  a  connection  with  the  11  telephone,  in  the  gate  house.  On  turning 
the  key  on  the  police-trip  box  the  door  swings  down  and  automatically 
sets  up  a  connection  with  police  headquarters.  Each  box  also  sends  in  a 
code  which  will  record  the  number  of  the  box.  As  soon  as  the  mechanism 
in  the  trip  box  comes  to  the  end  of  the  stroke  it  cuts  in  the  musolophone 
and  dictagraph  transmitter  so  that  it  is  then  possible  for  the  man  at 
headquarters  to  hold  a  conversation  with  the  party  at  the  box,  the  trans- 
mission being  commercial  within  a  radius  of  ten  feet.  A  howl  may  be 
set  up  in  the  horn  which  can  be  used  to  call  a  person  to  the  box. 

6.  EMERGENCY  SERVICE 

Industrial  plants  are  subject  to  happenings  other  than  fire  which  are 
classed  as  emergencies.  It  may  be  the  breaking  of  a  dam,  the  general 
failure  of  power  lines — anything  which  calls  for  prompt,  concerted  action 
directed  by  those  in  authority  who  are  best  able  to  lead  in  actions  which 
are  out  of  the  regular  routine. 

For  this  emergency  alarm  service,  some  number  like  99  or  999  is 
arranged  the  same  way  as  the  "11"  telephone  in  the  gate  house,  except 
that  instead  of  having  a  telephone  across  it,  there  is  a  high-voltage  relay 
which  operates  the  general  signal.  This  signal  should  have  no  connec- 
tion whatever  with  the  code-call  signals.  In  one  plant  it  is  a  big  steam- 
boat whistle. 

Red  cards  are  posted  throughout  the  plant  stating  that  whenever  the 
emergency  signal  is  heard,  all  unimportant  telephoning  must  cease  at 
once,  so  as  to  relieve  the  switches  for  certain  officials  whose  names  are 
given  on  the  card.  These  names  are  usually  limited  to  eight  or  ten, 
although  the  only  limit  is  the  number  of  selectors  or  connectors  which 
handle  the  t  raffic. 

When  any  person  becomes  aware  of  a  condition  which  justifies  the 
use  of  the  emergency  alarm,  he  goes  to  any  P.A.X.  telephone  and  calls 
the  general  alarm  number.     When  the  signal  sounds,  the  gate-house 


PRIVATE  AUTOMATIC  EXCHANGE  175 

man  or  some  other  designated  person,  gets  in  on  the  alarm  line  and  re- 
ceives the  report  of  the  man  who  sent  in  the  call.  At  the  same  time,  all 
the  officials  mentioned  on  the  red  card  who  hear  the  signal  call  go  to  the 
nearest  telephone  and  dial  the  alarm  number,  thereby  getting  in  on  the 
same  line  with  the  gate-house  man  and  the  person  who  reported  the 
trouble.  They  at  once  have  a  conference  as  to  the  steps  to  be  taken.  It 
gives  the  men  at  the  head  of  affairs  the  chance  to  know  what  is  going  on 
and  to  safeguard  the  business. 

When  those  on  the  emergency  line  hang  up  their  receivers  and  release 
the  general  signal  stops. 

7.  CREDIT  SYSTEM 

The  P.A.X.  affords  a  very  satisfactory  means  of  asking  for  and  trans- 
mitting information  concerning  credit  in  department  stores  and  other 
mercantile  establishments.  When  organizing  a  credit  system  as  many 
clerks  are  provided  as  may  be  needed  to  handle  the  traffic.  Each  clerk 
is  assigned  the  credits  relating  to  certain  positions  of  the  alphabet.  On 
each  credit  desk  is  provided  a  set  of  lamps  and  keys.  Instructions  similar 
to  the  following  are  given  to  all  sales  people: 

For  credits  A  to  H  dial  6. 
For  credit  I  to  M  dial  7. 
For  credit  N  to  Z  dial  8. 

A  Mr.  Kenyon  purchases  a  bill  of  goods  and  requests  credit,  the  store 
clerk  removes  the  receiver,  dials  7,  and  is  connected  with  the  credit 
clerk  who  handles  credits  under  the  letter  K.  Ten  trunks  may  be  pro- 
vided to  each  desk,  connections  may  be  held  and  the  credit  clerk  may 
ring  back  when  information  is  ready.  Should  Mr.  Kenyon  pass  on  to 
another  department  and  again  ask  for  credit,  the  store  clerk  will  dial  7 
and  be  connected  with  the  credit  clerk  who  handled  the  previous  call. 

Sometimes  the  head  of  the  credit  department  wants  to  give  all  the 
credit  clerks  immediate  and  simultaneous  instructions.  In  one  case 
there  were  nineteen  of  these  clerks,  and  time  is  an  important  matter.  A 
spare  number  on  the  P.A.X.  is  run  to  a  line  which  goes  to  the  desks  of  all 
these  clerks,  each  having  a  key  by  means  of  which  they  can  get  in  on  that 
line.  Across  the  line  is  a  relay  which  gives  a  signal  to  the  clerks  (green 
lamps  or  buzzers).  The  head  of  the  credit  department  dials  the  number 
and  the  clerks  switch  over  to  the  instruction  line,  leaving  their  bells  on 
their  regular  lines  for  incoming  calls. 

8.  SECRET  LISTENING  SERVICE 

In  treating  of  the  watchmen's  service,  mention  was  made  of  a  dicto- 
graph transmitter  placed  in  the  annunciator  cabinet,  so  that  the  chief  can 
by  calling  a  secret  number  be  connected  to  it  and  hear  all  that  is  going  on 
there.     This  same  idea  may  be  carried  out  wherever  desired.     The  trans- 


176  AUTOMATIC  TELEPHONY 

mitter  is  very  sensitive  and  will  pick  up  conversations  in  the  average 
tone  of  voice  within  the  ordinary  room.  The  transmitter  is  located 
according  to  the  conditions. 

It  is  sometimes  desirable  for  a  head  executive  to  know  what  is  going 
on  over  the  telephone  lines.  Such  a  case  is  met  by  providing  him  with  a 
switch  arranged  to  cut  in  on  any  line  regardless  of  the  busy  test.  The 
connection  is  arranged  to  come  in  quietly,  so  that  those  on  the  line  being 
monitored  are  not  aware  of  it. 

If  an  executive  receives  a  telephone  call  from  a  person  and  the  con- 
versation takes  such  a  turn  that  he  feels  the  need  of  a  witness,  he  can  by 
a  secret  signal  (buzzer,  etc.)  have  his  private  secretary  come  in  on  the  line 
by  the  monitor  switch  and  take  down  in  shorthand  the  conversation 
which  occurs.  The  same  restricted  monitoring  can  be  had  without  the 
switch  by  using  a  tap  taken  off  the  telephone  line. 

9.  DICTATION  SERVICE 

It  is  a  frequent  occurrence  for  a  man  who  is  out  somewhere  in  the 
plant  to  call  up  his  own  office  and  dictate  letters  or  memoranda  over  the 
telephone. 

Another  form  of  dictation  service  has  been  used  in  connection  with 
the  Edison  phonograph.  It  has  to  do  with  incoming  calls  during  the 
absence  of  the  owner  of  the  telephone. 


To  Maior 


Edison 
Phonoqraph 


Fig.    166. — Phonograph  to  answer  a  telephone  call  (circuit). 

First,  there  is  a  phonograph  on  which  is  recorded  the  record  which  it 
is  desired  that  the  calling  person  shall  hear.  It  may  be,  say  that  the 
manager  is  out  and  will  return  after  lunch,  or  it  may  direct  that  the  caller 
dial  another  number  and  leave  a  meassage  with  some  one  else. 

Second,  there  is  another  phonograph  arranged  to  receive  a  message 
from  the  telephone  which  is  calling. 

In  furnishing  the  dictating  service  Edison  machines  are  used.  These 
are  standard  machines  and  are  not  changed  in  any  way.  A  standard 
telephone  transmitter  is  mounted  adjacent  to  the  transcribing  head  of 
the  machine,  while  a  series  receiver  is  mounted  before  the  receiving 
head  of  the  machine.  In  connection  with  the  executive's  telephone 
number  79,  shown  in  Fig.  166,  is  a  two-point  switch  marked  D-T.  The 
switch  is  normally  in  the  T  position,  when  the  executive  leaves  his  office 
he  turns  the  switch  to  the  D  position.     This  remove's  the  short  circuit 


PRIVATE  AUTOMATIC  EXCHANGE  177 

from  relay  A,  and  should  a  call  come  in,  the  first  impulse  of  ringing  current 
will  operate  this  relay.  Relay  A  closes  the  circuit  of  relay  B,  relay  B, 
upon  energizing  locks  its  self  up  across  the  line  and  starts  the  Edison 
motor.  The  transmitter  will  now  reproduce  in  the  telephone  circuit  the 
words  spoken  into  it  by  the  transcribing  head  of  the  Edison  machine. 

The  message  on  the  record  may  be  a  standard  form  saying  the  execu- 
tive is  out,  and  requesting  the  calling  party  to  call  the  dictating  number, 
or  it  may  be  a  special  message  left  by  the  executive.  The  equipment 
for  the  dictating  number  77  is  shown  in  Fig.  167.  When  this  number  is 
called,  relay  A  operates  and  closes  the  circuit  of  the  Edison  machine. 
The  series  receiver  transfers  the  dictated  words  from  the  telephone 
circuit  to  the  receiving  head  of  the  machine.  A  set  of  springs  are 
mounted  in  place  of  the  bell  on  the  Edison  machine  so  that  when  it 
comes  to  the  end  of  a  record  a  buzzer  will  operate  which  will  indicate  to 

+■    -      a 

Switched  by  Carriage         \fj~fffji  I 


Connector  ^  ~l- 

Banks        -j  ^^  r->Kj    --^     To  Motor 

+  L      ^\ 1      I 11  ^  Phonograph 

Fig.    167. — Phonograph  to  receive  a  telephone  message  (circuit). 

those  seated  near  the  machine  that  a  new  record  is  needed,  this  buzzer 
produces  a  tone  on  the  line  which  advises  the  person  dictating  of  the 
fact  that  the  record  is  completely  filled. 

10.  MISCELLANEOUS  SERVICES 

An  electrical  house  which  sells  lamps,  fixtures,  etc.,  has  a  private 
automatic  exchange  with  a  number  of  vacant  call  numbers.  They  also 
have  about  30  demonstration  lamps,  chandeliers,  etc.,  which  they  desire 
from  time  to  time  to  light  for  the  benefit  of  customers.  They  arranged 
it  so  that  a  relay  on  each  of  these  vacant  lines  could  be  made  to  pull  up 
and  light  one  of  the  demonstration  lamps.  In  this  way,  from  any  auto- 
matic telephone,  the  salesman  could  show  off  his  wares. 

This  is  an  example  of  things  which  users  of  the  P.A.X.  have  put  upon 
it  without  in  the  least  interfering  with  its  intended  major  services. 
Others  have  used  a  spare  number  to  operate  an  electric  door  opener.  It 
is  beyond  the  function  of  this  book  to  enumerate  more  services — they 
can  be  thought  out  and  applied  whenever  the  need  arises. 

CONNECTIONS  TO  PUBLIC  TELEPHONE  SYSTEM 

There  are  many  conditions  under  which  it  is  desirable  to  have  trunk 
connections  between  the  private  automatic  exchange  and  the  public 
system.     It  then  becomes  an  automatic  private  branch  exchange.     Three 

12 


178  AUTOMATIC  TELEPHONY 

considerations  require  attention,  whether  the  public  exchange  is  manual 
or  automatic,  the  use  of  an  attendant  to  supervise  incoming  and  outgoing 
calls,  and  the  handling  of  toll  or  long  distance  calls.  We  will  first  assume 
that  the  public  exchange  is  automatic. 

Incoming  Trunk  Calls. — For  handling  incoming  trunk  calls  the  local 
switchboard  is  equipped  with  one  or  more  incoming  trunk  connector 
switches,  the  banks  of  which  are  multipled  with  the  banks  of  the  con- 
nectors used  for  local  interconnections.  The  incoming  trunks  to  these 
connector  switches  terminate  in  selector  switch  banks  at  the  central 
office.  Whether  they  terminate  in  the  banks  of  first,  second,  third,  or 
fourth  selectors  depends  upon  local  conditions.  If  the  central  office  is 
part  of  a  system  of  100,000  lines  ultimate  capacity  they  would  usually 
terminate  in  the  banks  of  either  third  or  fourth  selectors.  Suppose  for 
example  that  third-selector  switches  in  the  fifty-eighth  thousand  section 
of  the  central  office  are  decided  upon  in  a  given  instance  and  that  the 
first  level  of  those  banks  is  to  be  used  for  trunks  to  a  given  automatic 
branch  exchange.  Then  a  calling  party  would  have  to  call  "581"  to 
secure  a  trunk  terminating  in  an  idle  incoming  trunk  connector  switch 
at  the  automatic  P.B.X. 

To  complete  the  connection  to  the  P.B.X.  subscriber  he  must  also  call 
the  last  two  digits  of  the  subscriber's  local  number.  If  the  subscriber's 
local  number  were  "237"  it  would  appear  in  the  public  exchange  directory 
as  "58137"  and  any  subscriber  to  the  public  exchange  calling  that  num- 
ber would  secure  connection  to  it  just  as  if  it  were  connected  direct  to  one 
of  the  public  central  offices. 

The  circuit  for  such  a  connection  does  not  differ  in  any  way  from  that 
already  shown  in  Chapter  III  for  a  connection  passing  through  a  central 
office  connector  switch;  consequently  a  diagram  and  description  of  it  will 
not  be  given  here. 

Outgoing  Trunk  Connections. — The  outgoing  trunks  from  the  auto- 
matic P.B.X.  to  the  central  office  usually  terminate  in  first  selector  banks 
at  the  P.B.X.  and  in  line  switches  or  first  selector  switches  (generally  the 
former)  at  the  central  office.  Each  trunk  is  equipped  with  a  repeater  at 
the  P.B.X.  A  P.B.X.  subscriber,  when  calling  a  telephone  connected  to 
the  public  central  office,  does  so  in  the  usual  way  with  the  exception  that 
before  calling  the  subscriber's  number  as  it  appears  in  the  public  directory 
he  calls  a  preliminary  digit  which  was  decided  upon  when  the  P.B.X.  was 
installed  and  which  is  required  to  place  the  wipers  of  the  P.B.X.  first 
selector  in  connection  with  the  bank  contacts  of  an  idle  outgoing  trunk. 
For  example,  if  the  trunks  terminate  in  the  third  level  of  the  first-selector 
bank  contacts,  a  P.B.X.  subscriber  desiring  to  call  "2487"  would  dial 
"3-2487." 

Limited  Service. — It  frequently  is  desirable  to  limit  the  service  of 
certain  P.B.X.  stations  to  intercommunication  only.     Such  stations  may 


PRIVATE  AUTOMATIC  EXCHANGE 


179 


be  prevented  from  receiving  incoming  trunk  calls  by  disconnecting  their 
normals  from  the  connector-bank  multiple  of  the  incoming  trunk  connec- 
tor switches,  leaving  the  normals  connected  to  the  multiple  of  the  local- 
connector  switches  only;  or  a  station  which  is  not  to  receive  incoming 
trunk  calls  may  be  prevented  from  doing  so  by  having  its  private  bank 
contact  permanently  grounded  at  the  incoming  trunk-connector  switch 
bank  terminal.  This  causes  anyone,  who  attempts  to  call  that  station, 
to  receive  the  busy  signal.  They  may  be  prevented  from  making  out- 
going trunk  calls  by  the  use  of  a  special  type  of  repeater  on  the  outgoing 
trunks  and  the  use  of  a  350-ohm  resistance  coil  shunt  across  the  terminals 
of  the  bridge  cut-off  relay  winding  of  each  switch  which  is  to  be  allowed 
to  have  outgoing  trunk  service. 

The  circuits  of  a  line  switch  and  repeater  arranged  for  this  discriminat- 
ing service  are  shown  in  Fig.  168.  As  already  indicated,  the  lineswitch 
circuit  is  of  the  usual  type  with  the  exception  of  the  resistance  coil 


To  Phone 


-  Trunk 


X- Ad  justed  very  weak  Y  and  Z 

to  Close  onlu  when  line  sw.  has  350  to 

Coil  connected  accross  the  B.C.O.  coil 


Fig.   168. — Discriminating  outgoing  trunk  circuit. 


bridged  across  the  bridge-cut-off  coil  terminals.  The  repeater  has  the 
customary  double-wound  quick-acting  line  relay  L.R.,  slow  relay  S.R., 
condenser  cut-off  relay  C.C.O.R.,  and  trunk-holding  bridge  coil  B.R. 
The  release  trunk  instead  of  being  connected  direct  to  earth  by  a  contact 
controlled  by  the  slow  relay,  passes  through  the  winding  of  the  30-ohm 
discriminating  service  relay  D.S.R.  Since  this  relay  is  of  compara- 
tively low  resistance,  it  is  a  simple  matter  to  adjust  it  so  that  it  will  not 
pull  down  its  armature  when  the  lineswitch  connected  to  the  repeater 
does  not  have  the  350-ohm  shunt  across  the  terminals  of  the  bridge  cut-off 
winding,  i.e.,  it  is  adjusted  so  that  it  will  pull  down  its  armature  through 
350  ohms  and  1300  ohms  in  multiple  but  will  not  do  so  when  it  receives 
current  through  a  1300-ohm  resistance. 

If  this  relay  does  not  attract  its  armature  the  trunk  is  not  closed 
through  to  the  central  office  and  the  calling  party  receives  the  busy  signal 
because  the  springs  are  so  adjusted  that  the  relay  will  have  strength 
enough  to  close  the  contact  from  the  positive  side  of  the  trunk  to  the  busy 


180  AUTOMATIC  TELEPHONY 

bus  bar  although  it  does  not  have  strength  enough  to  close  the  trunk 
contacts. 

Automatic  Private  Branch  Exchanges  with  Supervised  Incoming 
and  Outgoing  Trunks. — Where  outgoing  trunk  calls  are  to  be  made  by  the 
users  of  the  automatic  private  branch  exchange  without  the  aid  of  an 
operator  or  where  subscribers  to  the  public  telephone  are  allowed  to  call 
1  he  various  private  branch  exchange  telephones  directly  without  an  opera- 
tor's assistance  provision  for  supervising  the  calls  going  in  either  direction 
may  be  very  simple  and  inexpensive — in  fact,  all  that  is  required  is  a  series 
relay  of  low  resistance  (about  10  ohms)  connected  in  one  side  of  each 
trunk  and  bridged  by  a  condenser.  If  the  springs  of  this  relay  are  ar- 
ranged to  close  the  circuit  through  a  supervisory  lamp  then  a  signal  light 
corresponding  to  each  trunk  will  glow  whenever  that  trunk  is  in  use. 
This  series  relay  should  be  slow  acting,  so  that  it  will  not  flutter  when 
ringing-  or  calling  device-impulses  are  passing  over  the  trunk. 

A  switch  should  be  arranged  so  that  the  pilot  lamps  can  be  cut  out  of 
circuit  except  when  they  are  required.  With  these  pilot  lamps  to  indicate 
when  connections  are  established,  a  key  associated  with  each  trunk  may 
be  used  to  bridge  a  receiver  in  series  with  a  ^  m.f.  condenser  across  the 
trunk  so  that  a  supervisor  can  hear  what  is  being  said  without  the  knowl- 
edge of  the  parties  talking.  If  desired  it  can  be  arranged  so  that  by 
throwing  the  same  key  in  the  opposite  direction  and  using  a  receiver  of 
the  direct-current  type  a  transmitter  can  be  connected  in  series  with  the 
receiver  and  the  condenser  shunted  out  so  that  the  supervisor  can  speak 
to  either  of  the  parties  on  the  line  if  desired. 

The  operator  should  always  throw  her  receiver  with  condenser  in  series 
on  to  the  line  first,  however,  so  that  if  the  calling  party  should  be  in  the 
act  of  setting  up  his  connection  to  the  called  party  she  will  not  interfere 
with  him. 

Trunk  Calls  set  up  by  an  Operator. — Where  it  is  desirable  to  install 
equipment  so  that  either  the  outgoing  or  the  incoming  trunk  calls  are  to 
be  set  up  by  an  operator  at  the  private  branch  exchange  more  elaborate 
provisions  than  those  mentioned  in  the  foregoing  paragraphs  are 
required. 

A  comparatively  simple  equipment  may  be  used,  however,  installed  in 
a  small  cabinet  of  the  cordless  manual  private  branch  exchange  type 
already  described,  and  all  switching  may  be  done  by  means  of  keys. 
The  circuits  for  calls  going  in  either  direction  may  be  arranged  so  that 
after  the  operator  has  set  up  the  connection  she  need  pay  no  further 
attention  to  it  because  the  ringing  will  be  done  automatically  by  the 
connector  switch  employed  and  when  the  parties  hang  up  their  receivers 
the  releasing  of  the  switches  used  in  the  connection  will  be  effected 
automatically. 

The  circuits  may  be  arranged  so  that  the  operator  can  listen  in  on  the 


PRIVATE  AUTOMATIC  EXCHANGE  181 

connections  after  she  has  set  them  up  or  so  that  it  will  be  impossible  for 
her  to  do  so. 

Incoming  Calls. — A  circuit  arrangement  for  an  equipment  of  this 
character  is  shown  in  Fig.  169.  This  diagram  illustrates  the  circuit  of  a 
non-secret  equipment  for  use  on  a  trunk  which  terminates  in  rotary 
connector  banks  at  the  central  office  and  in  a  lineswitch,  first  selector  or 
connector  switch  at  the  automatic  private  branch  exchange.  The 
connector  switch  used  for  this  circuit  at  the  private  branch  should  be  of 
the  reversing  battery  type  in  order  to  operate  the  supervisory  features. 

When  a  connector  switch  at  the  central  office  connects  to  this  trunk 
the  500-ohm  bridge  coil  is  energized  and  closes  the  circuit  from  earth 
through  the  trunk  signaling  lamp.  The  operator  responds  by  throwing 
the  key  in  the  direction  that  will  bridge  her  telephone  across  the  trunk 
and  close  circuit  from  earth  through  the  1300-ohm  release  control  relay. 
When  this  relay  attracts  its  armature  it  breaks  the  circuit  through  the 
signal  lamp  and  locks  itself  by  closing  circuit  to  earth  through  the  contact 
of  the  500-ohm  bridge  relay.  After  the  operator  has  taken  a  subscriber's 
number  she  throws  her  key  in  the  opposite  direction,  which  opens  the 
trunk  and  connects  her  calling  device  to  the  local  switchboard  end  of  it, 
then  operates  her  calling  device  to  call  the  particular  number  desired  and 
restores  her  key  to  normal.  The  connector  switch  rings  the  called 
party  automatically  and,  meanwhile,  supervision  is  furnished  and  the 
connection  as  established  is  prevented  from  releasing  by  the  bridge  across 
the  line  through  the  polarized  supervisory  relay  P.R.  When  the  called 
party  responds  and  the  direction  of  current  flow  is  consequently  reversed 
through  this  polarized  relay  the  circuit  through  the  supervisory  lamp  is 
broken  and  the  parties  proceed  with  their  conversation.  If  the  called 
party  should  wish  to  signal  the  operator  for  any  reason — for  example  to 
tell  her  switch  the  calling  party  to  some  other  local  station — he  can  secure 
her  attention  by  moving  his  receiver  switch  hook  up  and  down  slowly. 
This  causes  the  polarized  relay  to  make  and  break  the  circuit  through 
the  supervisory  lamp  thus  giving  the  usual  flash  signal  to  the  operator. 
The  operator  can  release  the  local  connection  at  any  time  when  her  calling 
device  is  in  circuit  by  simply  pressing  the  release  button  and  can  then  call 
another  party  without  interfering  with  the  incoming  connection.  After 
having  once  established  the  connection  the  operator  need  pay  no  further 
attention  to  it  unless  signalled  by  the  local  party. 

When  the  calling  party  releases  the  switches  in  the  central  office,  the 
switches  on  the  branch  switchboard  are  released  automatically  because 
when  the  circuit  through  the  bridge  relay  is  broken  by  the  release  of  the 
connector  switch  at  central  office  the  circuit  is  broken  through  the  1300- 
ohm  release-control  relay.  When  the  armature  of  this  relay  falls  back 
circuit  is  broken  through  the  polarized  supervisory  relay  and  as  a  result 
the  local  connection  immediately  releases. 


182 


A  U  TOM  A  TIC  TELEPHON  Y 


Night  Calls. — It  will  be  noted  that  a  night  key  is  provided  in  the  trunk 
and  that  when  it  is  thrown  it  cuts  out  the  operator's  equipment  and 


Telephone 


NIGHT  KEY 


I'l'l'l^! 


Fig.    169. — Circuit  through  an  automatic  P.B.X.  attendant's  cabinet  (non-secret). 


NIGHT  KEY 


Fig.   170. — Circuit  through  an  automatic  P.B.X.  attendant's  cabinet  (secret). 

connects  the  trunk  directly  to  a  line  which  may  lead  to  any  local  station. 
At  the  same  time,  it  disconnects  the  line  of  this  particular  telephone  from 
the  local  switchboard. 


PRIVATE  AUTOMATIC  EXCHANGE  183 

Another  arrangement,  similar  to  the  one  in  Fig.  169,  is  shown  in  Fig. 
170.  In  fact,  the  only  difference  between  the  two  is  that  the  latter  is 
"secret."  This  is  accomplished  by  making  the  trunk-key  connections 
such  that  when  the  operator  listens  in  on  the  incoming  section  of  the 
trunk  she  can  converse  with  the  calling  party  only.  When  she  throws 
the  key  in  the  opposite  direction  she  can  converse  with  the  called  party 
but  the  calling  party  is  cut  off.  With  this  exception  the  operation  of  this 
trunk  is  the  same  as  that  in  the  former  figure. 

In  many  cases  one  telephone  for  night  service  is  not  enough.  There 
may  be  no  one  near  it  all  the  time,  although  there  may  be  a  watchman 
who  is  making  the  rounds  of  the  building  all  night.  This  watchman  can 
be  reached  by  running  a  line  of  call  bells  so  as  to  get  the  sound  to  all 
places  where  he  will  be.  The  night  key,  shown  in  Figs.  169  and  170,  is 
arranged  to  switch  the  incoming  line  to  this  night-call  circuit.  A  vacant 
number  on  a  connector  bank  is  wired  to  this  circuit.  If  during  the  night 
a  call  comes  in,  it  is  diverted  from  the  attendant's  telephone  to  the 
night-call  circuit  and  rings  all  the  bells.  The  watchman  hearing  the  bells, 
goes  to  the  nearest  telephone,  dials  the  answering  number  (this  stops 
the  bells)  and  answers  the  caller  from  outside. 

Outgoing  Trunk  Calls. — Either  of  the  circuits  just  described  may  be 
reversed  and  thereby  used  equally  well  on  a  trunk  outgoing  from  a 
branch  automatic  switchboard  to  central  office.  The  operation  would  be 
the  same  as  that  just  described  with  the  exception  that  the  night  key  can 
not  be  used  and  should  not  be  installed  in  connection  with  an  outgoing 
trunk. 

If  the  trunk  coming  from  the  local  switchboard  terminates  in  selector 
banks  instead  of  in  connector  banks  then  the  incoming  trunk  portion  of 
the  operator's  equipment  should  be  arranged  as  in  Fig.  171  so  that  the 
double  wound  coil  will  supply  talking  current  to  the  calling  party  and 
will  connect  the  release  trunk  to  earth  so  as  to  hold  up  the  incoming 
connection  and  light  the  signal  lamp  until  the  operator  responds.  With 
the  exception  of  the  substitution  of  this  double  wound  relay  for  the  bridge 
relay  this  circuit  is  the  same  as  the  two  previously  described. 

As  a  rule  it  would  not  be  advisable  to  have  the  trunks  incoming  from 
the  public  exchange  to  equipment  of  this  character  terminate  in  selector 
banks  in  the  public  exchange  because  it  would  make  it  somewhat  more 
difficult  to  switch  the  trunks  through  to  the  telephone  for  night  service 
due  to  the  necessity  of  providing  some  means  for  ringing  the  night  tele- 
phone when  called. 

Because  an  operator's  equipment  using  any  one  of  the  circuits  just 
described  is  provided  in  connection  with  an  automatic  private  branch 
exchange  it  does  not  necessarily  follow  that  either  all  of  the  incoming  or 
all  of  the  outgoing  trunks  should  pass  through  it.  Sometimes  it  is  desir- 
able to  have  all  of  the  incoming  trunks  except  one  terminate  in  selector 


184 


AUTOMATIC  TELEPHONY 


banks  at  the  central  office  and  pass  directly  to  the  local  switchboard. 
The  line  excepted  may  terminate  in  a  regular  connector  multiple  at  the 
central  office  and  be  designated  under  its  proper  number  in  the  public 
directory  as  the  information  clerk's  telephone  of  the  establishment  in 
which  the  automatic  private  branch  exchange  is  installed.  With  this  ar- 
rangement subscribers  to  the  public  exchange  can  call  the  various  local 
stations  automatically  but  if  they  do  not  know  whom  to  call  or  wish  for 
any  reason  to  secure  the  services  of  the  attendant  they  call  the  number 
which  appears  in  the  directory  as  that  of  the  information  clerk  and  thus 
secure  connection  to  an  incoming  trunk  terminating  in  a  circuit  like 
that  shown  in  Fig.  169  or  170  in  the  attendant's  cabinet.  The  infor- 
mation clerk  responds  to  each  such  call,  gives  the  information  wanted, 
and  if  desired  sets  up  the  local  connection  for  the  calling  party. 


.Release 


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Release 


Fig.   171.- — Outgoing  trunk  circuit  through  attendant's  cabinet. 


A  similar  plan  may  be  used  on  outgoing  trunks,  the  arrangement 
being  such  that  certain  privileged  parties  may  make  all  outgoing  con- 
nections without  the  help  of  the  operator  while  the  others  can  make 
outgoing  connections  with  her  aid  and  approval  only. 

Apartment  House  Automatic  Private  Branch  Exchange. — This  is  a 
type  of  private  branch  exchange  which  has  been  developed  and  used  to  a 
large  extent  in  San  Francisco,  which,  especially  at  certain  seasons  of  the 
year  has  a  large  tourist  population.  For  the  accommodation  of  families 
of  tourists  this  city  contains  many  family  hotels  or  apartment  houses 
which  make  a  speciality  of  supplying  furnished  apartments. 

It  will  readily  be  understood  that  the  number  of  intercommunicating 
calls  between  the  occupants  of  the  apartments  of  any  building  will 


Pu%iutton  i-iniLamp  mn:  In^rt  StrapY.f  it      °PCfa1ori  LmeSwCkH9285       I   Operators MCkt.-IKT 5 

a  Desired  to  Put  Disconnect 
Supervision  and  Rdttse  Under 
Control  of  PA.  X.  Operator 
Fio.   172.— Circuits  from  manual  exchange  to  PA  X.  (PaeingP 


PRIVATE  AUTOMATIC  EXCHANGE  185 

generally  be  very  small  and  in  fact  almost  negligible  but  that  the  occu- 
pants will  desire  to  make  outgoing  trunk  calls,  to  receive  incoming  trunk 
calls,  and  to  communicate  with  the  janitor,  office  or  landlord  of  the  build- 
ing. It  has  therefore  been  found  that  equipment  most  suitable  for  these 
houses  is  similar  to  that  used  in  sub-offices  rather  than  that  used  in 
regular  automatic  private  branch  exchanges  and  that  using  the  sub- 
office  type  of  apparatus  facilitates  central  supervision  of  the  apartment 
house  apparatus. 

The  apartment  house  equipment  used  in  San  Francisco  has  been  quite 
fully  described  by  Mr.  Gerald  Deakin  in  a  paper  presented  by  him  at  the 
Pacific  Coast  Meeting  of  the  American  Institute  of  Electrical  Engineers  at 
Portland,  Oregon,  April  16-20,  1912,  and  published  in  full  in  the  trans- 
actions of  the  Institute. 

Connections  to  Manual  Public  Exchange. — When  the  public  exchange 
is  manual,  the  trunks  to  a  private  automatic  exchange  become  somewhat 
more  complicated  because  of  the  introduction  of  the  operator  at  the 
public  switchboard.  It  is,  of  course,  possible  to  let  these  trunks  be  in 
reality  subscriber  lines  as  far  as  the  P.A.X.  is  concerned,  and  to  let  the 
operators  in  the  public  exchange  dial  the  numbers  in  the  P.A.X.  directly. 
But  for  operating  reasons,  and  for  the  sake  of  a  certain  amount  of  control 
over  incoming  calls  to  the  P.A.X.  it  is  customary  to  have  an  attendant 
at  the  P.A.X. 

Provision  must  be  made  for  supervision  of  the  call  by  the  public 
exchange  operator,  for  answering  by  the  P.A.X.  attendant,  for  dialing 
calls  into  the  P.A.X.  by  the  attendant,  and  usually  for  using  the  same 
trunks  for  calls  to  the  public  exchange  as  well. 

The  circuits  shown  in  Fig.  172  are  arranged  for  a  three-digit  P.A.X. 
which  trunks  to  a  Kellogg  two- wire  switchboard.  A  call  from  the  manual 
board  to  the  automatic  board  is  intercepted  by  the  attendant  who  extends 
it  into  the  P.A.X.  by  means  of  a  calling  device.  The  manual  operator 
controls  the  release  and  gets  the  usual  supervision. 

Calls  from  the  P.A.X.  to  the  public  system  are  dialed  through  the 
selectors  only,  one  level  being  set  aside  for  it.  The  public  exchange 
operator  gets  a  line-lamp  signal,  plugs  into  the  jack,  and  receives  the 
number  from  the  subscriber.  The  seizure  of  the  trunk  by  the  selector 
gives  a  busy  signal  to  the  attendant.  The  P.A.X.  subscriber  controls  the 
release  of  the  connection,  and  the  manual  operator  gets  a  disconnect  signal. 

Manual  to  Private  Automatic  Exchange. — The  public  exchange 
operator  plugs  into  the  trunk  jack.  This  reverses  the  polarity  of  battery 
on  the  trunk  and  operates  the  12,000-ohm  polar  relay  P  which  is  nor- 
mally across  the  trunk  at  the  P.A.X.  Relay  P  operates  relay  C,  which 
grounds  the  private  wire  of  the  trunk  coming  from  selector  banks,  and 
displays  the  visual  busy  signal. 

The  operator  rings  on  the  trunk  as  usual.     This  operates  A.C.  relay 


18G  AUTOMATIC  TELEPHONY 

D,  which  lights  the  line  lamp  on  the  attendant's  cabinet  and  causes 
relay  H  to  lock  itself  to  ground  at  relay  C 

The  P.A.X.  attendant  answers  by  throwing  the  listening  key  on  the 
trunk.  This  grounds  the  operating  wire  to  the  incoming  selector  so  as 
to  prepare  it  for  dialing.  Relay  E  operates  and  locks  itself  to  ground  at 
relay  C,  puts  out  the  line  lamp,  releases  relay  H,  and  cuts  off  relay  D 
across  the  line.  At  the  same  time  polar  relay  Q  (500-ohms)  is  connected 
across  the  trunk,  to  give  supervision  to  the  manual  operator.  Polar 
relay  P  may  fall  back,  because  relay  Q  will  hold  a  ground  on  the  same 
wire. 

After  the  attendant  has  received  the  information  and  decided  whom 
to  dial,  she  throws  her  key  over  the  dialing  position  (marked  CD.) 
This  cuts  off  the  trunk  from  manual  and  connects  the  attendant  to  the 
incoming  selector  with  the  calling  device  inserted  in  the  operating  wire. 
The  connection  is  dialed  up  over  an  operating  wire  which  is  switched  to 
the  connector.  The  talking  wires  are  clear  from  the  battery  feed  in  the 
trunk  (relay  F)  to  the  called  telephone.  When  the  attendant  gets  the 
person  or  department  desired,  she  restores  her  key  to  normal  and  conver- 
sation proceeds. 

The  lamp  marked  "Supy"  on  the  trunk  burns  until  the  called  sub- 
scriber answers,  when  it  goes  out.  After  that  there  is  no  local  super- 
vision, the  public  operator  getting  it  by  the  bridging  of  polar  relay  Q. 

As  long  as  the  plug  is  in  the  jack,  polar  relay  P  or  Q  will  hold  the 
connection.  But  when  the  operator  pulls  out  the  plug  at  the  end  of 
conversation,  the  reversal  of  battery  on  the  trunk  causes  the  polar  relay 
to  reverse,  which  lets  relay  C  fall  back,  which  takes  ground  off  relay  E 
and  the  operating  wire  to  the  incoming  selector.  Then  the  automatic 
switches  release. 

P.  A.  X.  to  Manual. — The  subscriber  dials  one  digit,  which  puts 
the  wipers  of  the  selector  onto  the  out-trunk  level.  Current  is  at  once 
drawn  by  the  calling  telephone  through  relay  B,  which  operates  relay  L, 
causing  it  to  ground  the  release  trunk  to  hold  the  connection.  Relay  L 
pulls  up  relay  M,  which  operates  the  busy  visual  signal,  so  that  the  at- 
tendant will  know  that  this  trunk  is  busy.  Relay  B  also  connects  polar 
relay  Q  across  the  line  and  closes  the  talking  circuit  through  the  con- 
densers. 

The  500-ohm  path  through  relay  Q  operates  the  line  signal  in  the 
central  office.  The  operator  plugs  into  the  jack  as  usual.  This  reverses 
the  battery  on  the  trunk  and  pulls  up  relay  Q,  but  it  does  nothing  at  this 
time. 

The  number  is  passed  to  the  operator,  who  completes  the  connection 
in  the  usual  manual  manner. 

When  the  conversation  is  ended,  the  P.A.X.  subscriber  hangs  up  his 
receiver.     Relay  B  falls  back,  cutting  off  relay  Q  and  thus  giving  the 


PRIVATE  AUTOMATIC  EXCHANGE  187 

disconnect  signal  to  the  operator.  Relays  L  and  M  now  fall  back  and 
release  the  selector  and  restore  the  busy  visual  signal. 

There  is  only  one  mode  of  operation  as  far  as  the  P.A.X.  subscriber 
is  concerned — if  he  hangs  up  his  receiver,  the  connection  releases.  This 
is  true  if  he  makes  an  automatic  call,  and  if  he  makes  a  manual  call. 

Restricted  Service  to  Public  Exchange. — If  it  is  desired  that  not  all 
of  the  stations  in  a  P.A.X.  shall  be  permitted  to  call  out  into  the  public 
system,  it  may  be  accomplished  by  causing  the  rotary  magnet  of  the 
selector  to  depend  (at  a  certain  level)  on  a  ground  carried  from  the  line- 
switch  of  the  calling  telephone.  Those  telephones  which  are  permitted 
to  call  out  are  equipped  with  this  ground,  the  others  are  not. 

Figure  173  shows  a  selector  and  a  lineswitch  arranged  for  this  service. 
It  is  assumed  that  the  lineswitch  is  that  of  a  favored  telephone  and 
therefore  has  the  ground  marked  "M. " 

The  selector  is  equipped  with  a  set  of  springs  marked  "  X"  which  are 
set  in  relation  to  the  wiper  shaft  so  as  to  be  operated  when  the  wipers 
are  at  the  out  trunking  level  (level  ten,  for  example).  At  this  level  the 
ground  which  otherwise  would  go  through  the  off  normal  springs,  front 
contact  of  relay  B  and  back  contact  of  relay  C  to  the  rotary  magnet  is 
cut  off  and  a  busy  tone  sent  through  the  ground  winding  of  the  line  relay 
A  to  the  calling  subscriber. 

Since  this  lineswitch  has  the  ground  M ,  the  rotary  magnet  will  use  it 
and  cut  in  on  the  level.  The  busy  tone  will  be  cut  off  when  a  trunk  is 
found,  so  quickly  that  the  subscriber  is  ordinarily  not  aware  of  it. 

Telephones  whose  lines  witches  have  no  ground  M  can  call  all  other 
levels,  for  the  springs  X  are  not  operated.  But  if  they  attempt  to  call 
level  ten,  the  opening  of  the  ground  at  springs  X  prevents  rotation,  and 
the  busy  tone  is  left  on  until  the  calling  subscriber  hangs  up. 

Night  Calls. — The  night  key  (Fig.  172)  is  used  to  switch  a  trunk  to 
one  designated  night  telephone.  This  telephone  may  be  one  used  for 
regular  service  during  the  day,  and  can  not  be  said  to  differ  from  others. 
But  when  the  night  key  is  thrown,  it  will  receive  calls  over  this  trunk, 
though  it  can  make  calls  the  same  as  during  the  day. 

The  night  key  cuts  off  the  line  lamp  on  the  trunk  in  the  P.A.X., 
prepares  the  ring  side  of  the  line  through  the  supervisory  relay  J,  and 
connects  relay  K  to  the  private  normal  wire  which  runs  to  the  BCO  of  the 
lineswitch  which  belongs  to  the  night  telephone. 

When  the  operator  plugs  in  to  make  a  night  call,  the  battery  reversal 
on  the  trunk  operates  relay  P,  which  pulls  up  relay  C  to  busy  the  selector 
bank,  and  pulls  up  relay  A  which  switches  the  night  telephone  from  its 
lineswitch  to  the  trunk.     The  operator  then  rings  the  station  as  usual. 

When  the  night  telephone  answers,  it  operates  relay  J,  which  grounds 
relay  C,  because  relay  P  (12,000-ohms)  will  fall  back.  Supervision  to 
the  manual  board  is  direct. 


188 


AUTOMATIC  TELEPHONY 


A 


PRIVATE  AUTOMATIC  EXCHANGE  189 

If  the  night  telephone  initiates  a  call,  the  lineswitch  grounds  the 
private  normal  wire  PN  which  operates  relay  K.  The  latter  cuts  off 
relay  A  to  insure  the  circuit  against  being  cut  off. 

If  during  the  time  that  the  night  telephone  is  calling,  the  operator  in 
the  public  exchange  calls  the  P.A.X.,  relay  P  will  operate  relay  C,  which 
in  turn  gives  relay  N  a  chance  to  operate  periodically  through  the  con- 
tacts of  the  interrupter  Intr.  This  gives  a  busy  tone  to  the  operator, 
showing  that  the  line  is  busy. 

Toll  Connections  to  P.A.X. — If  the  public  exchange  is  manual,  the 
toll  connection  is  set  up  by  the  toll  operator  in  much  the  same  way  as  a 
local  call  is  handled.  The  trunk  conditions  can  be  arranged  so  as  to  give 
ample  transmission.  If  the  public  exchange  is  automatic,  it  is  best  to 
control  the  switches  over  a  third  wire,  so  as  to  leave  the  two  line  wires 
free  for  talking. 

Figure  174  shows  a  complete  typical  set  of  circuits  for  toll  and  local 
calls,  with  the  circuits  in  the  main  exchange  outlined.  If  we  trace  a  call 
from  a  toll  operator  to  a  P.A.X.  subscriber  it  will  show  the  operation  of 
the  circuits  and  open  the  way  for  further  study  by  the  reader. 

When  the  toll  operator  plugs  into  the  trunk  jack,  she  seizes  the  first 
selector  over  the  sleeve  of  the  circuit,  and  dials  the  connection  through. 
The  toll  third  selector  has  a  repeating  coil,  and  in  the  case  of  a  multi- 
office  public  exchange  is  located  in  the  office  to  which  the  P.A.X.  is 
tributary.  The  operating  wire  terminates  in  an  operating  delay  OpRy  in 
the  third  selector.  The  connection  passes  through  a  repeater,  in  which 
the  impulses  are  repeated  again  by  another  operating  relay,  OpRy. 
This  repeater  is  arranged  for  two-way  service.  Calls  from  the  P.A.X. 
to  the  public  exchange  pass  through  to  the  lineswitch.  When  the  re- 
peater seizes  the  trunk  in  obedience  to  the  toll  operator's  dialing,  the 
repeater  places  ground  on  the  private  wire  P  of  the  lineswitch,  operating 
the  BCO  and  cutting  off  the  line  relay  and  ground  of  the  lineswitch. 

The  repeater  is  seizing  the  runk  to  the  P.A.X.  grounds  the  operating 
trunk  wire,  which  pulls  up  the  operating  and  holding  relays  of  the  incom- 
ing selector  in  the  P.A.X.  They  busy  the  trunk  from  the  local  selectors 
(marked  "  10th"  level)  and  operate  the  visual  busy  signal  on  the  attend- 
ant's cabinet. 

The  last  three  figures  of  the  call  number  are  then  dialed  by  the  toll 
operator,  the  impulsing  being  done  over  the  operating  trunk  as  before. 

If  the  called  line  is  not  busy,  the  incoming  connector  will  go  to  the 
third  position  and  extend  the  talking  wires  through  to  the  called  tele- 
phone. From  the  repeating  coil  in  the  toll  third  selector  to  the  telephone 
there  is  no  bridge  or  break  except  the  two  condensers  and  feed  relay  G 
in  the  incoming  selector. 

When  the  incoming  connector  goes  into  the  third  position,  battery 
from  the  busy  common  is  placed  back  on  the  negative  line  for  an  instant, 


190  AUTOMATIC  TELEPHONY 

because  relay  D  is  slow  acting.  This  pulls  up  and  locks  relay  //  on  the 
incoming  selector.  Relay  F  on  the  selector  also  pulls  up  and  locks  when 
the  switch  goes  off  normal  by  means  of  the  ground  on  the  ring  through 
trunk. 

When  the  toll  operator  rings  out  on  this  connection,  ringing  current 
is  sent  through  the  condensers  of  the  incoming  selector  to  the  called 
telephone,  ringing  the  bell. 

When  the  called  person  answers,  relay  G  of  the  incoming  selector 
pulls  up  and  short  circuits  the  condensers  and  at  the  same  time  the  line 
relay  LR  in  the  toll  third  selector  pulls  up  and  gives  the  toll  operator 
supervision.  Current  supply  to  the  P.A.X.  telephone  is  double — the 
relay  G  of  incoming  selector  in  parallel  with  the  line  relay  LR  of  the  toll 
3d  selector. 

The  incoming  selector  in  stepping  off  normal  on  this  call  grounds  the 
supervisory  trunk  which  lights  the  supervisory  lamp  on  the  attendant's 
cabinet.  When  the  called  party  answers,  this  light  goes  out.  It  will 
follow  the  hook  the  same  as  the  supervisory  lamp  on  the  toll  board  so 
that  both  the  toll  operator  and  the  P.A.X.  attendant  have  supervision. 
If  it  is  desired  not  to  have  the  attendant's  lamp  show  until  the  called 
station  has  answered  and  hung  up  again,  it  may  be  secured  by  discon- 
necting the  battery  from  the  back  contact  of  relay  A  in  the  attendant's 
cabinet. 

The  toll  operator  releases  the  connection  by  pulling  the  plug  out  of 
the  jack.  The  removal  of  ground  from  the  operating  trunk  causes  the 
operating  relays  in  succession  to  fall  back  and  to  release  the  section  of 
the  connection  for  which  they  are  responsible. 

If  the  called  line  is  busy  on  the  call  which  we  have  been  considering, 
the  busy  tone  and  negative  battery  will  be  placed  back  on  the  positive 
line  at  the  busy  relay  E  of  the  incoming  connector,  which  will  result  in 
giving  the  toll  operator  a  busy  tone  and  the  supervisory  lamp.  If  the 
toll  operator  leaves  the  connection  up,  when  the  line  becomes  idle  the 
connector  will  move  to  the  third  position  and  extend  the  lines  as  explained 
before.  The  operator's  supervisory  lamp  now  burns,  showing  her  that 
she  may  ring  the  called  line. 

If  the  called  subscriber  desires  to  signal  the  attendant  to  get  the  call 
switched  or  otherwise  changed,  he  can  operate  the  hook,  which  will  flash 
the  attendant's  supervisory  lamp.  The  attendant  can  answer  by  the 
listening  key,  release  the  incoming  selector  and  connector,  with  the 
"release  in"  key,  and  dial  up  another  local  station  by  the  "CD.  in"  key 
and  the  calling  device.  In  this  case  the  P.A.X.  station  will  be  rung 
periodically  automatically.  But  after  this  second  party  has  answered 
and  hung  up  again,  he  can  only  be  rung  by  the  toll  operator  at  or. 

When  the  attendant  uses  the  "CD.  in"  key  for  dialing,  the  impulses 
pass  over  the  operating  trunk  with  200  ohms  in  series.     She  listens  with 


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PRIVATE  AUTOMATIC  EXCHANGE  191 

a  condenser  in  scries  with  her  telephone  set.  When  using  the  calling- 
device  keys  the  incoming  line  is  cut  off  so  that  she  talks  to  the  local 
station  alone. 

The  "call  through"  key  must  be  thrown  in  order  that  the  calls  from 
the 'outside  may  go  through  uninterrupted.  This  gives  negative  battery 
to  relay  E  of  the  incoming  selector  so  that  it  can  rotate  when  "it  arrives 
at  the  desired  level. 

If  it  is  desired  to  stop  all  calls  coming  in,  the  "call  through"  key  is 
left  at  normal.  Suppose  that  in  this  condition  a  call  comes  in  from  out- 
side. As  soon  as  the  incoming  selector  shaft  steps  off  normal,  the  ground 
that  ordinarily  operates  the  interrupter  relay  E  now  operates  relay  C  in 
the  attendant's  cabinet  and  lights  the  answering  lamp.  Relay  E  in  the 
selector  does  not  operate  and  the  shaft  does  not  rotate  at  all.  The  call 
can  go  no  farther.  The  operator  can  answer  by  using  the  listening  key, 
and  extend  the  call  by  the  "CD  in"  key  and  calling  device.  The  5000- 
ohm  resistance  coil  in  the  attendant's  circuit  is  to  furnish  a  return  path 
for  the  busy  tone  when  the  "CD.  in"  key  is  thrown. 

The  toll  operator  can  call  the  attendant  by  dialing  a  level  on  the  in- 
coming selector  which  is  set  aside  for  this  purpose.  The  P-2  contacts  of 
this  level  are  all  grounded  so  that  the  switch  will  rotate  off  the  bank  and 
open  the  switching  relay,  D.  At  this  level  a  set  of  springs  marked  "  10th 
level "  operate  and  put  relay  C  of  the  attendant's  cabinet  across  the  line 
with  a  condenser  in  series.  When  the  toll  operator  rings,  relay  C  operates 
and  gives  the  signal.  The  attendant  answers  with  the  listening  key, 
which  puts  out  the  lamp,  and  places  a  500-ohm  bridge  across  the  trunk 
which  is  locked  there  until  the  call  is  extended  or  answered,  or  if  the  call 
is  not  answered,  until  the  toll  operator  releases.  This  bridge  is  locked 
across  the  trunk  so  that  when  the  attendant  answers  a  toll  call,  supervi- 
sion will  be  given  to  the  toll  operator  as  soon  as  the  call  is  answered  and 
held  until  the  P.A.X.  station  has  been  dialed,  answers,  and  then  hangs 
up. 

This  same  trunk  handles  calls  from  the  public  automatic  exchange  as 
well  as  toll  calls.  The  local  selectors  come  to  the  repeater  in  a  different 
way,  and  the  repeater  automatically  furnishes  the  ringing  current, 
operates  a  quick-ring  cut-off,  reverses  battery  back  to  the  switches 
behind  it,  and  in  general  acts  as  the  meeting  point. 


CHAPTER  VII 
MEASURED  SERVICE  EQUIPMENT 

Measured  service  is  a  title  which  is  commonly  applied  to  service  which 
is  charged  for  in  accordance  with  the  rate  at  which  it  is  ussd  by 
the  customer.  Generally  the  charge  is  based  on  the  number  of  messages 
sent  by  each  customer,  no  attention  being  paid  to  the  number  of  messages 
received  by  him. 

There  is  no  scientific  reason  apparent  for  charging  a  fixed  rate  per 
message  for  local  service  in  automatic  telephone  systems.  Since  the 
connections  are  handled  by  machinery,  a  comparatively  wide  variation  in 
the  number  of  connections  per  line  per  day  for  which  apparatus  must  be 
provided,  makes  a  very  small  difference  in  the  first  cost  of  the  central 
office  equipment,  no  difference  whatever  in  the  cost  of  the  subscriber's 
station  and  line  equipment  and  makes  but  a  small  difference  in  the  cost 
of  keeping  the  apparatus  in  good  working  order. 

While  measured  service  decreases  the  traffic  and  consequently  permits 
a  reduction  in  the  trunk  lines  and  switches,  the  saving  in  the  installation 
and  maintenance  costs  of  the  trunking  equipment  is  largely  if  not  entirely 
offset  by  the  installation  and  maintenance  costs  of  the  equipment  required 
for  counting  the  calls  made  by  each  subscriber. 

The  money  spent  for  sub-station  meters  or  coin  boxes  would  produce 
a  better  effect  if  invested  in  additional  switches  at  the  central  office. 
The  cost  of  5000  coin  boxes  will  buy  about  1400  additional  selectors, 
which  would  double  the  number  of  selectors  in  the  average  5000-line 
office.  The  cost  of  central  office  meters  for  5000  lines  will  buy  about 
680  additional  selectors.  This  is  an  increase  of  50  per  cent  in  selectors 
which  will  carry  at  least  50  per  cent  more  traffic — perhaps  60  per  cent. 

It  seems  to  the  writers  that  to  induce  people  to  use  the  telephone 
freely  is  a  public  service  of  sufficient  value  and  importance  to  cause  the 
investment  of  money  in  traffic-carrying  apparatus  rather  than  in  that 
which  reduces  traffic.  It  would  therefore  appear  that  measured  local 
service  in  automatic  telephone  systems  must  be  justified  almost  entirely 
on  the  ground  of  expediency. 

Such  service  may  be  given  for  one  or  more  of  the  following  reasons: 

1.  To  satisfy  legal  requirements  or  to  comply  with  the  demands  of  a 
public  utility  commission. 

2.  As  a  means  of  gaging  the  charge  which  it  is  expedient  to  make  for 
each  patron's  service.     Telephone  rates  must  be  regulated  to  some  extent 

192 


MEASURED  SERVICE  EQUIPMENT  193 

by  charging  what  the  "traffic  will  bear."  This  is  generally  recognized  by 
charging  more  for  business  service  than  for  residence  service,  although  the 
average  first  cost  of  a  residence  line  is  considerably  more  than  the  average 
first  cost  of  a  business  line  and  the  difference  in  the  operating  cost  does  not 
justify  the  difference  in  the  rates  between  the  two. 

3.  Such  service  is  given  in  hotels,  railway  stations,  and  other  public 
places  where  the  telephones  are  installed  for  the  convenience  of  the 
general  public  and  where,  since  no  one  patron  can  be  expected  to  pay  for 
all  of  the  service,  each  must  be  charged  a  fee  for  his  connection. 

4.  It  affords  a  means  of  supplying  cash  service  to  patrons  whose 
credit  is  questionable. 

The  automatic  telephone  system  can  furnish  any  kind  of  measured 
service  which  is  desirable.  There  is  no  engineering  obstacle.  The 
use  of  flat  rates  or  of  measured  service  is  entirely  a  matter  of  business 
policy. 

Measured  service  at  once  divides  itself  into  two  classes,  credit  service 
and  cash  service.  Credit  service  requires  that  the  amount  of  service 
sold  shall  be  recorded.  Cash  service  imposes  the  more  difficult  task  of 
collecting  money  for  the  service  before  the  service  is  rendered.  Thus 
the  two  requirements  vitally  affect  the  design  of  apparatus. 

Individual  lines  may  have  the  measuring  apparatus  in  the  central 
office  or  at  the  sub-station.  Party  lines  require  that  this  apparatus  be 
located  at  the  sub-station. 

The  following  combinations  of  conditions  have  been  met  by  devices 
for  use  in  connection  with  automatic  exchanges. 

For  credit  service  we  have: 

1.  Meter  at  central  office,  records  completed  calls. 

2.  Meter  at  sub-station,  operated  by  push-button  to  record  completed  calls. 

3.  Meter  at  sub-station,  self-acting,  to  record  completed  calls. 

4.  Meter  at  sub-station,  records  time  line  is  used  (cumulative  time  meter). 

For  cash  service  we  have  : 

5.  Coin  box,  coin  deposited  when  called  station  answers,  collected  at  once. 

6.  Coin  box,  coin  deposited  before  dialing,  collected  when  the  called  station 
answers. 

7.  Coin  box,  automatic    and  manual,    local  calls  free,  toll  calls  cash  (audible 
signal  to  operator). 

8.  Coin  box,  automatic  and  manual,  coin  deposited  before  dialing,  collected  or 
returned  on  release  of  connection,  toll  calls  cash  (audible  signal  to  operator). 

Central  Office  Meters.— Where  meters  are  used  in  central  offices  in 
systems  of  the  Automatic  Electric  Company's  manufacture  they  are  so 
arranged  that  there  is  a  meter  associated  with  the  line  switch  of  each  line 
on  which  the  service  is  to  be  measured.  When  a  subscriber  makes  a  call 
his  meter  does  not  register  it  until  the  called  party  responds  and  it  does 
not  register  even  then  if  the  subscriber  has  called  a  long-distance  oper- 

13 


194 


AUTOMATIC  TELEPHONY 


ator,  an  information  or  complaint  operator,  the  wire  chief,  manager  or 
some  other  employe  of  the  company.  Sometimes  it  is  arranged  also  so 
that  the  meter  will  not  register  if  the  subscriber  calls  the  police  depart- 
ment or  fire  department. 

In  Fig.  175  is  shown  a  photograph  of  a  lineswitch  unit  on  which  the 
meters  are  seen  mounted  in  a  group  above  the  lineswitches.  The  mechan- 
ical design  of  the  meters  is  the  same  as  that  of  those  which  are  widely 

used  in  manual  practice. 

A  circuit  showing  a  lineswitch  with 
meter  is  illustrated  in  Fig.  176.  The 
only  difference  between  the  lineswitch 
and  those  described  heretofore  is  the 
addition  of  a  pair  of  springs  closed  by 
the  plunger  when  it  is  drawn  down. 
The  operation  of  the  circuit  in  so  far 
as  the  meter  is  concerned  is  as  follows: 
When  the  calling  subscriber  lifts 
his  receiver  from  the  switch-hook,  the 
lineswitch  plunger  enters  its  bank  and 
extends  the  circuit  to  a  selector 
switch  in  •  the  usual  way.  As  the 
plunger  moves  toward  its  pole  piece 
it  closes  circuit  through  the  outside 
winding  of  the  meter  from  negative 
battery  to  the  private  normal,  which  is 
of  course  connected  to  earth.  At  the 
same  time,  however,  circuit  has  been 
closed  through  the  inside  winding  of 
the  meter,  which  is  connected  in  series 
in  the  negative  side  of  the  trunk  to  the 
connector  and  is  therefore  energized 
by  the  current  flowing  through  the 
line  relay  windings  of  the  connector 
and  the  subscriber's  loop.  At  this 
two  windings  oppose  each  other  so 
When  the  called  party  re- 


Fig.  175.- 


-Lineswitch  unit  with  meters 
at  top. 


stage   of    the    connection    these 

that  the  meter  armature  is  not  attracted. 

sponds,  however,  and  his  connector  reverses  the  direction  of  the  current 

flow  in  the  calling  party's  loop  then  the  inside  meter  winding  assists  the 

outside  winding  and  the  meter  armature  is  drawn  down  causing  the  meter 

to  register  and  shunting  out  the  inside  winding. 

The  outside  winding  retains  the  armature  until  conversation  is 
completed  so  that  the  calling  party's  talking  circuit  is  left  entirely  clear 
the  same  as  when  no  meter  is  used. 

If  the  calling  party  instead  of  setting  up  a  connection  with  another 


MEASURED  SERVICE  EQUIPMENT 


195 


subscriber  calls  some  number  which  he  is  to  be  afforded  free  service — for 
example  the  long-distance  operator — his  meter  does  not  register  because 
the  equipment  is  arranged  so  that  the  trunk  to  her  position  terminates  in 
the  banks  of  a  selector  switch  and  does  not  pass  through  a  connector 
switch,  and  her  cord  circuits  are  designed  so  that  when  she  responds  to 
his  signal  she  does  not  reverse  the  direction  of  current  flow  in  his  loop. 
Therefore  the  two  meter  windings  continue  to  oppose  each  other  and  the 
meter  does  not  register,  neither  does  it  short-circuit  its  inside  winding. 
This  winding,  however,  is  of  very  low  resistance  and  while  its  impedance 
is  almost  negligible,  it  may  be  reduced  almost  to  nil  by  shunting  it  with  a 
condenser,  a  non-inductive  resistance  coil  or,  better  and  simpler  still, 
by  placing  a  thin  pure  copper  sleeve  over  the  core  of  the  meter. 

Free  service  may  be  given  on  connections  which  pass  through  con- 
nector switches  also  by  segregating  such  lines  and  connecting  them  to  the 


METER 
Inside 
pN\  Winding. 


Open  Main 
(-Battery) 

Fig.   176. — Circuit  of  lineswitch  with  meter. 


banks  of  a  group  of  connectors  which  do  not  reverse  the  direction  of  cur- 
rent flow  in  the  calling  party's  loop  when  the  called  party  responds.  It  is 
readily  apparent  that  these  connectors  would  be  the  same  as  those  shown 
in  the  diagram  excepting  a  very  slight  difference  in  the  connecting  of  the 
back-bridge  relay  springs. 

If  it  is  desired  to  place  the  meters  at  a  distance  from  the  lineswitch 
boards,  grouping  them  all  at  one  place,  a  relay  takes  the  place  of  the 
meter,  but  is  placed  in  the  trunk  circuit.  This  relay  merely  closes  a 
single  wire  circuit  leading  through  the  lineswitch  bank  to  the  meter 
itself,  which  now  has  only  one  winding.  Thus  it  requires  but  one  wire 
per  meter  from  the  lineswitch  boards  to  the  meter  board. 

Push  Button  Meter  at  Sub-station. — This  type  of  meter  is  essentially  a 
counting  device,  operated  by  a  push  button,  with  means  for  requiring 
the  subscriber  to  register  when  the  called  station  answers,  and  safeguards 
against  registering  more  than  once  for  such  a  call. 


19G 


AUTOMATIC  TELEPHONY 


The  meter  (Fig.  177)  has  a  train  of  counting  wheels  whose  numerals 
appear  at  a  rectangular  window.  A  push-button  at  the  lower  right  hand 
corner  of  the  box  is  the  subscriber's  means  of  control. 

Inside  the  meter  there  is  the  counting  mechanism  (5  numeral  wheels) 
a  group  of  electrical  contact  springs,  and  a  polarized  electromagnet 
whose  winding  is  shunted  by  a  condenser. 

In  normal  operation,  the  subscriber  dials  the  desired  number  in 
the  usual  way.  When  the  called  station  answers,  the  connector  switch 
reverses  the  current  fed  to  the  calling  line.  This  energizes  the  polarized 
electromagnet  in  the  meter  and  causes  it  to  close  the  three  contact 


Fig.   177. — A  subscriber's  push-button  meter. 

springs.  By  them  a  dead  short  circuit  is  placed  on  the  transmitter 
and  a  low  resistance  (30-ohms)  shunted  around  the  receiver. 

The  calling  subscriber  can  hear  the  called  party  as  he  answers,  but 
cannot  speak  to  him.  Pressing  the  button  moves  the  counting  wheel 
and  frees  the  three  springs.     Conversation  may  now  proceed. 

If  the  push  button  be  pressed  when  the  telephone  is  not  in  use,  no 
record  is  made,  because  normally  the  pawl  which  works  the  recorder  is 
out  of  place  and  can  not  engage  the  wheel.  It  requires  the  movement 
of  the  polarized  magnet  to  bring  the  pawl  into  line  with  the  wheel,  so 
that  the  pressing  of  the  button  can  rotate  the  number  wheel. 

When  a  call  has  once  been  recorded,  the  pawl  is  again  thrown  out  of 
line,  so  that  further  actuation  of  the  button  will  have  no  effect. 


MEASURED  SERVICE  EQUIPMENT 


197 


All  conversations  are  held  through  the  electromagnet  and  the  con- 
denser. The  magnet  is  wound  to  16  ohms  and  the  condenser  has  two 
microfarads  capacitance. 

Free  calls  do  not  require  registry,  because  the  battery  current  is 
not  reversed,  as  was  explained  above. 

This  meter  requires  four  wires  between  itself  and  the  telephone  set. 

Self-acting  Meter  at  Sub-station. — The  self-acting  meter  (Fig.  178) 
appears  like  a  cylinder  with  a  window  at  which  appear  the  numbers  on  the 
register  wheels.     It  requires  four  wires  to  connect  it  to  the  telephone  set. 

The  subscriber  dials  the  number  as  usual.  When  the  called  station 
answers,  the  reversal  of  battery  current  actuates  the  meter  so  that  the 
pawl  is  in  a  position  to  move  the  number  wheel  when  release  shall  occur. 
The  electromagnet  is  short  circuited  during  conversation.     When  release 


Fig.  178. 

occurs,  the  apparatus  restores  itself  to  normal.  In  so  doing,  the  pawl 
turns  the  number  wheel  so  as  to  record  the  call. 

In  its  normal  condition  the  electromagnet  is  short  circuited.  When 
receiving  a  call,  the  coil  is  therefore  not  in  the  circuit.  If,  however,  the 
dial  should  be  turned  while  the  receiver  is  off  the  hook,  the  electromagnet 
will  come  into  the  line  in  series  with  the  talking  apparatus. 

On  free  calls,  the  electromagnet  is  in  series  with  the  line  and  conversa- 
tion must  be  held  through  it.  Its  resistance  is  40-ohms,  and  it  is  shunted 
by  a  half  microfarad  condenser.  Since  free  calls  are  never  set  up  over 
very  long  distances,  the  transmission  is  not  much  of  a  factor.  On  all 
calls  where  transmission  counts,  the  coil  is  short-circuited  and  therefore 
does  not  affect  speech. 

The  user  might  attempt  to  avoid  registry  by  momentarily  depressing 
the  hook  before  the  called  station  answers.  He  would  expect  by  the 
momentary  opening  of  the  line  to  cause  the  meter  operating  magnet  to 
restore  the  pawl  to  normal  before  it  had  caught  the  next  tooth  on  the 


198 


AUTOMATIC  TELEPHONY 


number  wheel.  This  would  be  successful  if  it  were  not  for  a  slow-acting 
element,  which  requires  that  the  line  be  held  open  long  enough  to  cause 
release  of  the  switches.  The  dishonest  person  soon  learns  that  he  obtains 
nothing  by  this  procedure. 

Cumulative  Time  Meter  at  Sub -station. — The  cumulative  time  meter 
is  essentially  a  clock  controlled  by  an  electromagnet  so  as  to  run  only 
while  a  completed  call  is  held.  In  appearance  it  is  a  rectangular  box, 
with  a  clock  face  appearing  near  the  lower  end.     (Fig.  179.) 

There  are  two  varieties.  The  first  counts  time  only,  the  second  makes 
an  arbitrary  charge  of  one  minute  at  the  beginning  of  each  call.  The 
latter  is  to  enable  the  operating  company  to  collect  part  of  its  return  in 
proportion  to  the  number  of  completed  calls,  and  the  rest  in  proportion 
to  the  time  of  occupation  of  apparatus. 


Fig.   179. 


When  the  subscriber  takes  the  receiver  from  the  hook  to  make  a 
call,  it  is  necessary  to  turn  a  knob  on  the  meter  box.  This  takes  the 
short  circuit  off  the  calling  device.  He  now  dials  the  call.  When  the 
called  station  answers,  the  reversal  of  current  actuates  a  polarized 
magnet  which  starts  the  clock  and  short-circuits  the  relay. 

If  a  fixed  charge  of  one  minute  is  made,  the  electromagnet  advances 
the  clock  hands  one  minute  at  the  time  of  starting. 

The  clock  will  run  until  the  calling  subscriber  hangs  up  his  receiver. 
Anything  that  the  called  party  may  do  will  not  affect  the  clock,  because 
the  polarized  electromagnet  is  shunted  out. 

When  the  calling  subscriber  hangs  up  his  receiver,  the  mechanism 
restores  itself  to  normal,  stopping  the  clock. 


MEASURED  SERVICE  EQUIPMENT 


199 


Received  calls  do  not  disturb  the  meter  and  there  is  no  added  appara- 
tus through  which  conversation  must  be  held.  Free  calls  do  not  affect 
the  clock  and  conversation  must  be  held  through  the  electromagnet 
and  its  shunt. 

Coin  Collectors  for  Subscribers  Stations. — Figure  180  is  a  photo- 
graphic reproduction  of  two  views  of  a  coin  collector — one  with  the  cover 
on  and  one  with  the  cover  removed — of  a  type  that  is  employed  to  a  con- 
siderable extent  in  automatic  exchanges. 

Figure  181  (right)  is  a  diagram  of  the  circuit  of  this  collector  in  con- 
nection with  the  ordinary  two-wire  wall  telephone.     The  essential  feature 


Fig.   ISO. — -Interior  and  exterior  views  of  automatic  coin  collector. 


of  the  collector  is  a  polarized  relay  P.R.  which  through  its  armature  arm 
and  a  trigger  controls  three  contact  springs  which  are  so  connected  up 
that  when  they  are  closed  together  they  short-circuit  the  telephone 
transmitter  and  place  a  low  resistance  shunt  across  its  receiver.  The 
trigger  is  arranged  so  that  it  will  be  tripped  when  a  suitable  coin  or  token 
passes  through  the  coin  chute  to  the  coin  box.  The  windings  of  the 
polarized  relay  P.R.  are  connected  in  series  in  one  side  of  the  subscriber's 
loop,  but  in  order  that  they  may  not  reduce  transmission  they  are  of  very 
low  impedance  and  bridged  by  a  2  m.f.  condenser.  To  reduce  the  imped- 
ance to  the  lowest  possible  amount,  a  copper  sleeve  is  placed  over  the 
core  of  each  coil. 


200 


A  UTOMA  TIC  TELEPHON  Y 


The  operation  of  the  mechanism  as  a  subscriber  makes  a  call  is  as 
follows: 

When  the  subscriber  lifts  his  receiver  from  the  switch-hook,  current 
flows  from  the  central  office  through  his  loop  energizing  the  polarized 
relay  windings  and  causing  the  armature  to  swing  its  arm  away  from  the 
coin  chute.  The  subscriber  proceeds  to  call  the  party  he  wishes  in  the 
customary  manner  and,  when  the  called  party  responds,  the  direction 
of  the  current  flow  is  reversed  in  the  calling  party's  loop  by  the  action  of 
the  reversing  battery-connector  switch  with  the  result  that  the  polarized 
relay  P.R.  swings  its  arm  in  the  opposite  direction,  i.e.,  toward  the  coin 
chute  and  thereby  causes  its  trigger  to  draw  the  three  shunt  springs  into 
contact  with  each  other  thus  short-circuiting  the  transmitter  and  placing 
the  30-ohm  shunt  across  the  receiver  terminals.  All  of  this  happens  in  an 
instant  so  that  when  the  called  party  speaks  into  his  transmitter  in 


line  < 


Fig.    181. — Two  different  circuits  of  a  telephone  with  a  coin  collector. 


response  to  the  call  the  calling  party  is  able  to  hear  him  but  can  not  talk 
to  him.  The  calling  partly  immediately  deposits  the  required  coin  in 
the  coin  chute,  however,  and  as  it  drops  to  the  coin  box  it  strikes  the 
trigger,  knocking  it  out  of  engagement  with  the  lever.  The  shunt  springs 
spread  apart  and  the  calling  party  is  enabled  to  converse  with  the  called 
party  in  the  usual  way. 

Should  the  calling  party  receive  the  "busy"  signal  or  receive  no 
response  whatever  to  his  call  he  saves  the  coin  by  not  depositing  it.  The 
purpose  of  the  low-resistance  shunt  placed  across  the  receiver  terminals 
is  to  prevent  the  calling  party  defeating  the  purpose  of  the  collector  by 
using  his  receiver  as  a  transmitter. 

Calls  to  long-distance  operators,  information  operators  and  other  tele- 
phone company  employees  may  be  made  and  conversation  carried  on 
without  the  deposit  of  a  coin  because,  as  explained  in  the  description  of 


MEASURED  SERVICE  EQUIPMENT  201 

the  meter  circuits,  arrangements  are  made  so  that  the  direction  of  the 
current  flow  in  the  calling  party's  loop  is  not  reversed  during  conversa- 
tion, consequently  the  polarized  relay  P.R.  of  the  collector  does  not  close 
the  shunt  springs. 

Figure  181  (left)  is  a  diagram  of  a  wall  telephone  connected  to  a 
collector,  the  circuit  of  which  is  slightly  different  from  that  just  described. 
The  special  feature  of  this  collector  is  that  it  gives  a  "tick  tick"  signal  to 
a  party  called  by  another  party  from  a  telephone  equipped  with  a  col- 
lector, for  the  purpose  of  reminding  the  called  party  that  he  should 
respond  to  the  call  by  giving  his  own  name,  the  name  of  the  company  or 
firm  to  which  the  called  telephone  belongs,  or  preferably  the  number  of 
the  called  telephone  so  that  the  calling  party  will  be  sure  that  he  has 
made  his  call  correctly  and  has  the  number  that  he  wishes  before  he 
deposits  the  coin  in  the  collector. 

The  mechanical  features  of  this  collector  are  the  same  as  those  in  the 
one  previously  described  but  when  the  polarized  relay  swings  its  arm 
toward  the  coin  chute,  thus  drawing  the  shunt  springs  together,  it  short- 
circuits  its  own  windings  at  the  same  time  that  it  places  a  shunt  across 
the  calling  party's  transmitter.  When  short-circuiting  takes  place  the 
polarized  relay  arm  immediately  swings  back  to  original  position  opening 
its  shunt,  and  then  immediately  swings  toward  the  coin  chute  again. 
Thus  the  polarized  relay  arm  is  moved  back  and  forth  repeatedly  making 
and  breaking  the  contact  between  the  shunt  springs  so  that  a  moderately 
loud  "tick  tick"  is  heard  by  the  called  party  due  to  the  shunting  in  and 
out  of  the  transmitter  and  polarized  relay  windings.  When  the  calling 
party  drops  the  required  coin  the  trigger  is  tripped  and  the  circuits  of  the 
telephone  are  left  clear  for  conversation. 

Coin  Box,  Coin  Deposited  Before  Dialing,  Collected  on  Answer. — It 
has  been  found  that  sometimes  the  calling  party  is  a  little  slow  in  deposit- 
ing the  coin  when  the  called  station  answers.  This  might  result  in  the 
called  subscriber  hanging  up  and  causing  the  calling  party  to  call  again. 
People  ought  to  have  the  nickel  ready  and  ought  to  drop  it  in  the  slot 
promptly. 

To  obviate  this  apparent  difficulty,  the  coin  box  (Fig.  180)  has  been 
arranged  to  induce  the  user  to  deposit  a  nickel  before  dialing,  so  that  it 
will  be  ready  for  instant  collection.  The  coin  is  held  there  in  plain  view, 
where  it  can  be  removed  by  the  subscriber  if  the  called  station  fails  to 
answer,  or  if  the  called  line  is  one  to  which  service  is  free. 

The  operation  is  as  follows.  The  user  first  places  a  nickel  in  the 
coin  slot  where  it  rests.  He  then  dials  the  number.  When  the  called 
station  answers,  the  coin  box  removes  the  support  for  the  coin,  whereupon 
it  drops  into  the  chute,  trips  the  springs,  and  passes  into  the  cash  com- 
partment. 

If  the  coin  is  not  collected,  the  user  recovers  it  by  hand. 


202 


A  UTOMA TIC  TELEPHONY 


Calls  can  be  made  without  the  preliminary  deposit  of  a  coin,  in 
which  case  the  payment  must  be  made  when  the  called  station  answers. 

Pay  Station  for  Long  Distance  and  Local  Service. — It  is  the  general 
practice  to  install  in  hotel  lobbies  and  other  places,  from  which  a  con- 
siderable number  of  long-distance  calls  may  be  expected  to  emanate, 
common  battery  manual  telephones  equipped  with  coin  collectors 
arranged  with  chutes  for  coins  of  three  or  four  sizes,  each  chute  being 
equipped  with  a  suitable  device  for  giving  a  signal  to  the  operator  who  is 
supervising  the  deposit  of  the  coins.     The  lines  from  these  pay-station 


Fig.   1S2. — Three-slot  eoin  box  telephone. 


telephones  are  connected  to  the  long-distance  board  and  are  handled  by 
the  operators  of  that  board.  Connections  to  long-distance  lines  or 
other  pay-station  lines  are  put  up  manually  and  connections  from  pay- 
station  lines  to  subscriber's  automatic  stations  are  set  up  by  using  a 
calling  device  installed  in  the  operator's  position  for  the  purpose. 

It  has  been  found  desirable  in  some  locations  however  to  install  auto- 
matic telephone  pay  stations  instead  of  manual  telephone  pay  stations 
and  to  equip  the  station  with  a  coin-collecting  device  which  may  be 
cither  for  collecting  the  coin  deposited  for  a  local  call,  which  is  not 
supervised,  or  for  a  long-distance  call  on  which  the  operator's  aid  and 


MEASURED  SERVICE  EQUIPMENT  203 

supervision  is  employed.  Such  a  collector  usually  has  three  slots — one 
for  5-cent  pieces,  one  for  dimes  and  one  for  quarter  dollars. 

Coin  Box,  Automatic  and  Manual,  Local  Free,  Toll  Cash. — This 
service  requires  no  electrical  connection  between  the  telephone  and  the 
coin  box.  Any  desired  box  is  mounted  so  that  its  audible  signals  (bell 
or  gong)  will  affect  the  transmitter.  More  than  one  type  has  been  used 
in  this  way. 

Local  calls  are  dialed  as  if  there  were  no  coin  box. 

To  call  long  distance,  the  user  dials  "0"  or  such  other  number  as  is 
indicated  by  the  directory  or  information  card.  The  operator  may  put 
the  call  through  while  the  subscriber  waits,  or  may  ask  for  his  number  and 
call  him  when  the  line  is  ready.  In  either  case,  the  operator  requests 
that  the  coin  be  deposited,  and  verifies  the  correctness  of  payment  by  the 
audible  signals. 

Coin  Box,  Automatic  and  Manual,  Nickel  First,  Toll  Cash. — This 
coin  telephone  (Fig.  182)  is  built  as  one  unit.  Three  slots  are  provided. 
The  deposit  of  any  coin  gives  a  distinctive  bell  or  gong  signal  and  in 
addition  prepares  the  line  for  dialing.  No  coin  is  necessary  to  receive  a 
call. 

To  make  a  call,  a  coin  is  first  deposited  in  the  appropriate  slot.  The 
number  is  then  dialed  as  usual.  When  the  called  subscriber  answers, 
conversation  takes  place.  When  the  calling  subscriber  hangs  up  to 
release,  the  machine  drops  the  coin  into  the  cash  compartment. 

If  the  call  is  unanswered,  or  is  to  a  telephone  for  which  calls  are  free, 
the  release  action  of  the  subscriber  causes  the  coin  to  be  dropped  into  the 
return  pocket  where  he  can  get  it  again. 

Long  distance  calls  pass  through  the  hands  of  an  operator,  who  calls 
for  the  deposit  of  the  required  fee  in  the  usual  manner. 


CHAPTER  VIII 
AUTOMATIC  TRAFFIC  DISTRIBUTOR  EQUIPMENT 

While  traffic  distributor  equipment  is  composed  of  automatic  switch- 
ing mechanisms,  it  is  used  to  increase  the  efficiency  of  manually  operated 
switchboards  and  not  to  give  automatic  telephone  service.  This  appa- 
ratus can  be  employed  in  full  automatic  systems  but  by  itself  does  not 
make  automatic  connections  between  subscribers.  It  is  used  only  for 
distributing  the  traffic  among  "A"  or  "B  "  manual  switchboard  operators 
in  such  a  manner  that  their  efficienc}^  is  greatly  increased.  Considerable 
of  the  increase  in  efficiency  of  semi-automatic  or  automanual  systems 
(automatic  switchboards  worked  by  operators)  is  due  to  the  traffic  dis- 
tributor features  which  form  a  part  of  these  systems,  and  is  by  no  means 
all  due  to  the  fact  that  the  operators  set  up  the  calls  by  means  of  push- 
button calling  devices  instead  of  by  means  of  cords  and  plugs. 

The  same  or  similar  traffic  distributor  features  may  be  and  have  been 
applied  equally  well  to  offices  in  which  the  operators  complete  the  connec- 
tions by  using  cords  and  plugs. 

Reasons  for  Loss  of  Efficiency  in  Manual  Offices. — There  are  four 
principal  difficulties  in  the  way  of  bringing  the  work  of  manual  switch- 
board operators  up  to  maximum  efficiency. 

First. — The  wide  variations  in  the  traffic  load  during  the  hours  of 
each  day. 

Second. — The  variations  between  positions  in  the  hour  that  the 
heaviest  traffic  occurs. 

Third. — The  momentary  traffic  rushes  and  corresponding  compara- 
tively idle  moments  which  are  especially  aggrevated  in  small  trunk 
groups. 

Fourth. — Since  it  is  impossible  to  supply  each  operator  with  a  constant 
stream  of  work  at  the  maximum  rate  at  which  she  can  handle  it,  it  is  very 
difficult  if  not  impossible  to  apply  the  bonus  or  premium  method  of  regu- 
lating her  wages  which  efficiency  engineers  have  learned  is  an  essential  to 
high  efficiency  on  the  part  of  most  wage  earners. 

Taking  up  the  discussion  of  these  four  difficulties  in  detail  and  in  order 
it  should  be  said : 

First. — The  manual  switchboard  operator  works  at  her  greatest 
efficiency  during  the  busy  hours  when  the  calls  are  coming  in  at  their 
maximum  rate  for  the  day.  In  fact  the  total  number  of  positions  which 
snail  be  equipped  in  a  manual  switchboard  is  calculated  by  dividing  the 
busy  hour  calls  for  the  whole  office  by  the  busy  hour  calls  which  one 

•204 


AUTOMATIC  TRAFFIC  DISTRIBUTOR  EQUIPMENT 


205 


operator  handles.  As  the  load  on  the  board  falls  off  each  operator  has 
less  to  do.  Consequently  telephone  companies  attempt  to  maintain  the 
efficiency  of  their  operators  by  reducing  the  number  at  times  of  light 
load.  This  is  termed  "adjusting  the  operators  to  the  load  curve." 
This  method  is  obviously  cheaper  than  maintaining  the  full  force  all  day. 
Yet  it  leaves  much  to  be  desired.  When  an  operator  has  to  handle 
more  than  one  position  she  can  not  answer  calls  so  fast  since  she  must 
reach  further  and  with  more  effort  on  each  connection.  The  reduction 
of  efficiency  is  clearly  shown  by  the  curve  in  Fig.  183. 

Starting  with  a  standard  of  100  per  cent  as  the  load  which  she  can 
handle  at  one  position,  she  can  care  for  only  73  per  cent  as  many  calls 
when  two  positions  are  assigned  to  her.  For  night  work  when  one 
operator  must  tend  many  positions, 
the  efficiency  is  very  low.  Ten  posi- 
tions give  us  a  load  only  18  per  cent 
of  her  full  one-position  ability.  Thus 
the  expedient  of  adjusting  the  number 
of  operators  to  the  load  results  in 
great  loss  of  efficiency  without  making 
the  work  any  easier  on  the  girls. 

Second. — The  busy  periods  do  not 
occur  at  exactly  the  same  time  on  all 
positions.  This  causes  a  further  loss 
in  efficiency.  Figure  184  was  taken 
from  an  actual  peg  count  and  illustrates 
the  inequality  very  well.  From  6  to 
7  a.m.  position  9  is  the  only  one  to 
have  an  appreciable  load.  From  10 
to  11  a.m.  positions  4,  6,  and  9  have  an  increased  load,  while  5,  7,  8, 
and  10  have  very  much  less  to  do.  The  afternoon  peak  comes  between 
4  and  5  on  positions  5  and  9,  between  5  and  6  on  positions  7  and  10,  while 
it  is  as  late  as  between  7  and  8  on  positions  4  and  8. 

In  general  the  traffic  manager  aims  to  rearrange  the  lines  at  the  inter- 
mediate distributing  frame  so  that  so  far  as  possible  the  busy  hour  load 
of  the  office  will  be  evenly  divided  among  the  operators.  This  distribu- 
tion is  a  matter  of  difficulty  for  it  requires  constant  attention  and  much 
thought  and  labor.     Very  few  exchanges  are  successful  in  securing  it. 

Third. — There  is  another  great  loss  of  efficiency  due  to  the  evil  of 
"rushes."  For  instance  when  we  say  that  225  calls  were  handled  by 
one  operator  in  one  hour  we  have  only  a  partial  idea  of  her  speed.  Dur- 
ing that  hour  the  calls  did  not  come  to  her  in  an  even,  steady  stream. 
There  were  periods  of  rush  when  she  may  have  been  answering  calls  at 
the  rate  of  300  or  400  per  hour  followed  by  short  periods  of  slow  calling 
or  even  idleness.     Formerly  the  only  known  method  of  reducing  the 


90 

80 

70 

3  60 

o 

i-  50 

z 

u    ,  - 

30 

20 

' 

2       3       4        5       6        7       8 
POSITIONS  HANDLED  BY  ONE  OPERATOR 


9       10 


Fig.  183. — Curve  showing  relative 
efficiencies  of  a  manual  operator  covering 
various  numbers  of  positions. 


206 


AUTOMATIC  TELEPHONY 


inequality  was  "team  work."  Each  operator  is  trained  to  keep  a  lookout 
over  the  position  to  her  right  and  left  so  that  if  her  neighbor  has  more 
than  she  can  do  assistance  can  be  given.  Though  this  reduces  the  evil  a 
little,  it  still  fails  to  get  at  the  root  of  the  matter.  The  wide  variations 
which  take  place  in  the  traffic  passing  over  small  trunk  groups  as  com- 
pared with  large  groups  are  shown  in  the  chapter  on  traffic.  (Fig.  305.) 
These  curves  clearly  indicate  that  a  moderately  steady  flow  of  traffic  can 


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Fig.   1S4. — Simultaneous  load   curves  of  manual   switchboard   operators'   positions. 


be  expected  only  from  groups  made  up  from  large  numbers  (several 
thousand  or  more)  of  lines. 

Fourth. — Efficiency  engineers  have  found  that  a  workman  who  is 
making  or  assembling  the  same  piece  over  and  over  in  a  factory,  or  a 
bricklayer  who  is  applying  mortar  and  laying  bricks  over  and  over  all 
day  long  can,  in  most — if  not  all  cases,  be  induced  to  work  at  his  highest 
efficiency  only  when  (a)  scientific  standards  of  attainment,  determined 
by  proper  motion  studies  and  studies  of  conditions  are  placed  before  the 
workman  to  be  striven  for;  (6)  the  materials  for  his  work  are  supplied  to 


AUTOMATIC  TRAFFIC  DISTRIBUTOR  EQUIPMENT  207 

him  in  such  a  way  that  they  are  always  available  at  the  proper  time,  at 
the  proper  place,  and  in  the  proper  condition;  and  (c)  his  wages  are 
materially  increased  when  he  brings  his  work  up  to  or  within  striking 
distance  of  standard. 

Contrast  these  conditions  with  those  of  an  operator,  who  is  assigned  an 
arbitrary  group  of  lines  to  serve,  and  who  can,  therefore,  handle  the  calls 
only  as  the  erratic  load  fluctuations  allow  her  to  do  so. 

A  fifth  difficulty  is  found  in  the  way  to  high  efficiency  of  "  B  "  operators 
due  to  the  fact  that  if  the  number  of  trunks  between  the  "A  "  operators 
in  one  office  and  the  "B"  switchboard  in  another  is  sufficiently  great  to 
require  the  services  of  two  or  more  "B"  operators,  and  the  "A"  operators 
are  consequently  provided  with  order  wires  to  each  of  the  "B"  operators 
so  that  in  the  event  that  the  " B"  operator  regularly  assigned  to  an  "A  " 
operator  is  busy  she  can  switch  her  call  to  another  operator,  it  has  been 
found  that  it  is  not  unusual  for  the  "A  "  operators  to  distribute  the  work 
unevenly  among  the  " B"  operators.  This  is  due  to  two  reasons;  in  the 
first  place  the  arbitrary  assignment  of  order  wires  among  the  "A" 
operators  may  be  at  fault;  secondly  "A"  operators  sometimes  become 
convinced  that  some  particular  "B"  operator  gives  them  better,  atten- 
tion than  the  others  do,  with  the  result  that  they  are  inclined  to  send 
calls  through  to  the  favored  operator  which  should  go  to  other  operators 
and  thus  overload  her  while  her  mates  are  not  working  at  their  full 
capacity. 

The  Ideal  Condition. — It  is  apparent  from  the  foregoing  paragraphs 
that  to  secure  the  ideal  condition  of  maximum  efficiency  in  manual 
operation  it  is  necessary  to  supply  the  work  during  all  hours  of  the  day  to 
each  operator  on  duty  in  her  own  position  so  that  she  never  has  to  reach 
into  the  positions  on  either  side  of  her;  to  supply  the  same  amount  of  work 
to  her  during  each  hour  of  the  day  and  during  each  moment  of  each  hour 
to  supply  the  work  at  the  maximum  rate  at  which  she  can  handle  it 
properly,  without  excessive  nervous  or  physical  strain;  and,  in  the  case 
of  "B"  operators,  to  make  it  impossible  for  the  "A"  operators  to  show 
favoritism.  The  nature  of  telephone  traffic  is  such  that  it  is  impossible 
fully  to  realize  all  of  these  conditions,  but  several  kinds  of  equipment 
which  have  been  more  or  less  successfully  used  to  secure  the  ideal  con- 
dition will  be  described  in  the  remaining  portion  of  this  chapter. 

Christensen's  Electropneumatic  Selector. — This  selector  which  is  the 
invention  of  Mr.  P.  V.  Christensen,  assistant  chief  engineer  of  the  Copen- 
hagen Telephone  Company,  Copenhagen,  Denmark  in  1904  is  used  in 
Scandinavia  as  an  auxiliary  to  manual  switchboards  to  distribute  the 
load  among  "A"  operators  and  among  "B"  operators.  The  system 
for  promoting  better  distribution  among  "A"  operators  is  called  the 
"Operator's  Aid  System"  and  is  an  application  of  the  Christensen  selec- 
tor devised  by  Director  Fr.  Johannsen  of  the  Copenhagen  Telephone 


208 


AUTOMATIC  TELEPHON  Y 


Company,  while  the  system  for  securing  better  distribution  of  the  work 
among  "£"  operators  is  called  the  "Automatic  Order  Wire  System." 


Fig.  185.- — View  from  below  of  three  Chris-     Fig.   186. — View  from  above  of  Christensen 
tensen  pneumatic  selectors.  pneumatic  selectors. 


Fig.   187. — Racks  with  Christensen  pneumatic  selectors  mounted  upon  them. 

This  selector,  which  belongs  to  the  line  switch  type,  differs  from  other 
well-known  selectors  both  in  its  construction  and  in  its  method  of  opera- 


AUTOMATIC  TRAFFIC  DISTRIBUTOR  EQUIPMENT 


209 


tion.     The  motion  is  rectilinear  (not  rotating)  and  the  selectors  are 
constructed  to  find  an  idle  line  among  twenty  lines. 


Compressed  Air 


Fig.    188. — Christensen  pneumatic  selector  moving. 

Figure  185  is  a  view  of  the  selectors  from  below,  and  Fig.  186  is  a 
view  from  above.  For  comparing  the  dimensions  an  ordinary  slide  rule 
is  shown. 


1  1  1K(H) 


Epp 


Searching 


Fig.    189. — Christensen  pneumatic  selector  "searching." 

Figure  187  shows  the  iron  framework  with  the  selectors  arranged  in 
horizontal  rows,  each  row  having  space  for  twenty. 

Figures  188  to  190  are  sketches  showing  the  working  method  of  the 
selector. 


Fig.   190.- — Christensen  pneumatic  selector  "connecting." 

Figure  191  is  a  diagram  of  the  selector  parts  and  circuits. 

The  motive  power  consists  of  compressed  air  at  two  atmospheres  or 
2  kg.  per  sq.  cm.  (29  lbs.  per  sq.  in.)  and  is  automatically  maintained 
by  a  reserve  at  an  excess  pressure  of  0.9  to  1.1  kg.  per  sq.  cm.     A  two- 

14 


210 


AUTOMATIC  TELEPHONY 


coiled  electromagnet  A  opens  a  valve  B  which  admits  the  compressed 
air  into  a  cylinder  and  causes  a  piston  to  move  onward  (Fig.  188).  The 
search  mechanism  with  the  search  magnet  C  and  the  searching  contact 
roller  D  besides  the  necessary  contact  springs  E  are  attached  to  the  piston 
rod.  The  return  motion  takes  place  by  the  aid  of  a  watch  spring  F  when 
the  valve  closes  off  the  compressed  air. 

In  most  selector  constructions  the  motion  ceases  when  tne  motive 
power  (electrical  current)  is  cut  off,  but  in  this  selector  the  motive 
power  (air  pressure)  continues  while  the  motion  is  stopped  by  a  mechani- 
cal catching  device.  This  is  arranged  in  such  a  way  that  the  air,  when 
the  motion  ceases,  presses  the  moving  contact  springs  E  against  the  fixed 
contact  pieces  H  belonging  to  the  talking  and  signalling  wires  of  the 
selected  line.  When  the  searching  contact  roller  D  slides  over  a  search 
contact  K  which  is  in  connection  with  an  idle  line,  this  contact  being 
grounded,  the  search  magnet  will  attract  its  armature.  This  will  cause  a 
pin  L  which  is  connected  by  the  means  of  an  elastic  link  system  M  to 
the  search  magnet  to  be  raised  up  into  a  slanting  notch  in  the  brass  rail 


MWWU 


wwwvn 

100  U>        I 


50  uo 


]      1  H 

-<°X    O    O    O    O    O   O    O   OK 

D  I 


Fig.   191. —  Diagram  of  circuit  of  Christensen  pneumatic  selector. 


N  (Fig.  189).  Here  the  pin  catches  and  while  the  air  continues  to  press 
on  the  piston  the  link  system  will  yield  and  cause  the  movable  contact 
springs  E  to  be  pressed  upward  against  the  fixed  contact  pieces  of  the 
contact  strips  H,  by  a  pressure  of  200  gr.  per  contact.  The  contact 
springs  will  thus  connect  the  contact  strips  with  the  selected  wire's  bank 
contact  pieces  (Fig.  190)  and  the  circuit  desired  will  be  completed. 

When  the  controlling  magnet  A  is  deprived  of  current  the  valve  B 
closes  off  the  compressed  air  and  the  watch  spring  F  draws  the  piston 
back.  When  returning  the  pin  will  be  prevented  from  catching  in  the 
notches  in  the  brass  rail  TV  by  the  shape  of  the  notches. 

In  case  of  the  selector  not  finding  an  idle  line  at  first,  a  contact  is 
arranged  in  the  circuit  of  the  magnet  A  which  causes  the  selector  to  go 
backward  and  forward  until  it  finds  an  idle  line.  The  speed  of  the 
selector  is  such  that  but  0.6  second  is  required  for  an  investigation  of 
20  lines. 

The  amount  of  air  necessary  is  very  small  as  each  selector  consumes 
only  about  one  liter  of  air  at  atmospheric  pressure  per  busy  hour. 


AUTOMATIC  TRAFFIC  DISTRIBUTOR  EQUIPMENT 


211 


The  entire  movable  part  of  the  mechanism  can  be  exchanged  in  about 
35  seconds  should  any  complications  arise. 

The  Automatic  Order  Wire  System.— This  was  made  in  1907.  Figure 
192  is  a  diagram  of  the  circuit  of  the  Christensen  selector  employed  on 
order  wires  from  "A"  operators'  positions  to  "B"  operators'  positions. 
The  selector  is  also  used  between  "A"  operators'  positions  and  toll- 
recording  positions,  between  "A"  operators'  positions  and  information 
operators,  etc. 

The  operation  of  the  circuit  between  "A"  and  "B"  operators  is  as 
follows:  When  the  "A"  operator  presses  her  order  wire  key  she  closes 
circuit  from  battery  through  the  relay  U  and  the  two  100-ohm  windings 


Order  Wire  Key 
A  -  Position 


ft     A  -  Position 


Fig.   192. — Circuit  of  automatic  order  wire  system. 

of  electromagnet  A  to  earth.  Relay  U  disconnects  the  operator's  tele- 
phone circuit  from  the  listening  keys  of  her  cord  circuits  thus  automat- 
ically cutting  off  the  waiting  subscriber  while  a  "B"  operator  is  selected 
and  given  the  subscriber's  order.  The  electromagnet  A  admits  air  to  the 
piston  of  the  selector,  the  contact  fingers  of  which  immediately  move 
forward  over  the  bank  contacts.  As  soon  as  contacts  corresponding  to 
an  idle  order  wire  are  found  the  search  magnet  C  is  energized  through 
circuit  from  earth  through  one  of  the  switches  T  the  back  contact  of  the 
100-ohm  relay  S,  corresponding  to  the  particular  order  wire  on  which  the 
searcher  stops,  the  bank  multiples  of  this  order  wire,  the  corresponding 
search  contact,  the  roller  of  the  searcher,  winding  of  C,  the  other  roller  of 
the  searcher  and  thence  to  battery  through  the  winding  of  relay  U. 


212  AUTOMATIC  TELEPHONY 

The  energization  of  C  causes  it  to  attract  its  armature  which  instantly 
stops  the  searcher  and  presses  the  contact  fingers  down  on  to  the  banks. 
The  order  wire  seized  is  immediately  guarded  against  seizure  by  another 
searcher  because  the  corresponding  guard  relay  S  attracts  its  armature 
and  breaks  the  circuit  from  earth  through  the  search  contact  multiple  of 
the  order  wire.  The  circuit  through  S  is  from  earth  through  the  winding 
of  S,  the  contact  finger  of  the  searcher,  the  winding  of  relay  R  to  battery. 

While  the  searcher  is  in  motion  a  buzzer  signal  is  given  to  the  "A" 
operator  from  the  buzzer  current  generating  machine  P  through  the  back 
contacts  of  relay  R  and  the  contacts  of  the  order  wire  key,  but  as  soon  as 
the  searcher  finds  an  idle  order  wire  and  R  is  energized  through  the  circuit 
just  described  the  buzzing  stops,  the  operator  thereby  knows  that  she  is  on 
an  idle  order  wire  and  she  immediately  transmits  the  order  to  the  " B" 
operator  who  assigns  an  idle  trunk  in  the  usual  way.  When  the  "A" 
operator  releases  her  order  wire  key  the  electromagnet  A  will  release  the 
air  and  the  selector  will  return  to  normal  position  and  relay  S  will  again 
connect  the  search  contacts  to  ground.  Each  "B"  operator  can  be  easily 
cut  off  at  any  time  by  throwing  the  switch  T  of  the  corresponding  order 
wire. 

With  this  arrangement  each  operator  needs  but  one  order  wire  key  for 
each  office  to  which  she  trunks  connections.  The  efficiency  of  the  "A" 
operators  and  the  "  B"  operators  is  considerably  increased  and  the  over- 
loading of  "B"  operators  is  made  practically  impossible. 

Operators'  Aid-system. — The  "Aid-system,"  made  in  1909,  which 
can  be  used  not  only  in  new  but  also  in  old  exchanges  employing  either 
magneto  or  common  battery  switchboards  of  the  double  cord  type,  was 
designed  to  lighten  the  burden  of  the  "A"  operators  in  busy  moments. 
It  requires  only  slight  alterations  in  the  switchboard.  As  installed  in 
Copenhagen,  three  of  the  answering  cords  in  each  operator's  position 
are  connected  with  automatic  selectors,  and  in  this  way  changed  into 
"transfer  cords."  When  an  operator  uses  one  of  these  cords  to  respond 
to  a  call,  the  call  is  automatically  transferred  to  a  disengaged  colleague 
by  the  automatic  selecting  device,  which  can  search  for  an  idle  operator 
in  a  group  of  as  many  as  twenty  operators.  Even  in  extremely  busy 
moments  the  probability  of  finding  every  operator  engaged  in  the  group 
of  twenty  is  small,  and  consequently  the  transferred  subscriber  will 
quickly  secure  attention. 

For  receiving  "transfer  calls"  there  are.  four  "receiving  cords"  in- 
stalled in  each  position.  Both  the  transfer  and  the  receiving  cords  are 
of  the  single  type,  or  in  other  words,  when  a  transfer  has  been  completed 
the  transfer  cord  and  plug  used  to  answer  the  subscriber  constitute  one 
end  of  a  complete  cord  circuit,  and  the  receiving  cord  and  plug  used  to 
complete  the  connection  to  the  called  party  constitute,  for  the  time  being, 
the  other  end  of  the  same  cord  circuit. 


AUTOMATIC  TRAFFIC  DISTRIBUTOR  EQUIPMENT  213 

When  a  selector  transfers  a  connection  to  an  idle  cord  in  an  idle  opera- 
tor's position,  a  lamp  associated  with  the  cord  lights,  calling  the  attention 
of  the  operator  at  that  position  to  the  transferred  call.  The  detailed 
circuits  of  the  system  as  installed  in  Copenhagen  are  shown  in  Fig.  193. 

If  subscriber  A  should  remove  his  receiver  from  the  switch-hook  to 
make  a  call,  current  will  flow  through  his  loop  and  line  relay  L.R.,  which 
will  close  the  circuit  through  the  line  signal  lamp  1.  When  the  operator 
responds,  by  inserting  the  plug  3  in  the  jack  2,  current  will  flow  from 
earth,  through  one  winding  of  the  repeating  coil,  the  cord  relay  12,  tip 
of  the  plug,  tip  spring  of  the  jack,  the  subscriber's  loop,  the  ring  spring 
of  the  jack,  ring  of  the  plug  and  the  other  winding  of  the  repeating  coil 
to  battery.  When  relay  12  operates  it  will  close  circuit  from  battery, 
through  relay  13  to  earth.  At  the  same  time  the  cut-off  relay  C.O.R.  of 
the  line  will  cut  off  the  line  relay  because  it  will  be  energized  through 
circuit  from  earth,  through  winding  of  C.O.R. ,  sleeve  of  the  jack,  the  sleeve 
of  the  plug,  lamp  4  and  the  contact  of  relay  13  in  multiple  to  battery. 
At  the  same  time  that  relay  12  closes  the  circuit  through  the  relay  13,  it 
will  also  close  circuit  through  the  double  wound  electromagnet  9  of  the 
pneumatic  selector.  The  operation  of  this  electromagnet  will  admit  air 
to  the  cylinder  of  the  selector,  which  will  shove  the  piston  head  forward 
and  cause  the  search  spring  11  to  move  over  the  search  contacts  until  an 
idle  contact,  that  is,  a  contact  connected  to  earth,  is  found.  Then  a 
circuit  will  be  closed  from  battery  through  the  search  magnet  winding  10, 
search  spring  11,  search  contact,  the  break  contacts  of  relays  14,  15  and 
16,  and  the  switch  17  to  earth.  Search  magnet  10  will  immediately 
press  the  three  contact  fingers  down  so  that  each  will  close  circuit  between 
its  respective  rail  and  the  bank  contact  corresponding  to  the  idle  search 
contact.  The  two  sides  of  the  talking  circuit,  shown  in  heavy  lines  in 
the  drawing,  will  thereby  be  extended  through  to  the  receiving  cord  plug 
5.  At  the  same  time  circuit  will  be  closed  from  battery,  which  is  con- 
nected to  the  middle  rail  of  the  bank  through  the  contact  controlled  by 
relay  12,  through  relay  14  and  relay  21  in  series  to  earth.  While  relay 
14  is  operated,  circuit  will  be  closed  through  the  lamp  6,  corresponding  to 
the  receiving  cord,  which  signals  the  operator  at  the  position  in  which  the 
receiving  cord  is  placed.  When  the  operator  inserts  plug  5  into  the 
jack  of  the  line  leading  to  the  telephone  B  of  the  party  to  be  called,  cur- 
rent will  flow  from  earth  through  the  cut-off  relay  COR  of  the  called  line, 
the  sleeve  of  the  jack,  the  sleeve  of  the  plug,  relay  15  and  the  lamp  8  to 
battery.  When  "15"  operates,  it  breaks  the  circuit  through  "14," 
which  in  turn  breaks  the  crcuit  through  lamp  6.  Lamp  8  glows  until 
the  called  party  responds,  when  the  talking  current  operates  the  cord 
relay  20,  which  shunts  out  lamp  8.  When  the  calling  subscriber  hangs 
up  his  receiver,  relay  12  will  release  its  armature  and  then  relay  14  will 
again  attract  its  armature,  being  energized  by  circuit  from  earth  through 


214 


AUTOMATIC  TELEPHONY 


AUTOMATIC  TRAFFIC  DISTRIBUTOR  EQUIPMENT  215 

relay  13,  the  middle  rail  and  the  corresponding  bank  contact  of  the 
selector  to  the  winding  of  relay  14  and  the  make  contact  of  relay  15  to 
battery.  This  will  cause  the  lamp  6  to  light  again  and  give  the  disconnect 
signal  to  the  operator  at  the  receiving  cord  position. 

When  the  plug  5  is  removed  from  the  jack,  the  relay  15  will  drop  its 
armature,  breaking  the  circuit  through  14,  and  at  the  same  time  breaking 
the  circuit  through  the  electromagnet  9,  and  as  a  consequence,  the  piston 
of  the  selector  will  return  to  normal  position.  If  the  called  party 
hangs  up  his  receiver  before  the  plug  is  withdrawn  from  the  jack,  the 
lamp  8  will  give  the  disconnect  signal  as  soon  as  the  armature  of  the 
cord  relay  20  falls  back. 

It  was  stated  that  the  selector,  when  searching,  would  stop  on  a  cord, 
the  search  contact  of  which  was  connected  to  earth  through  the  back 
contact  of  its  relay  16.  The  wiring  of  this  relay  in  each  position  is  such 
that  if  any  listening  key  (7)  in  a  position  is  thrown,  the  earth  connection 
made  thereby  will  complete  the  circuit  through  "16"  and  cause  it  to 
break  the  earth  connection  of  all  search  contacts  corresponding  to  receiv- 
ing cords  in  that  position.  By  this  feature  the  selector  is  prevented  from 
transferring  a  call  to  an  operator  who  is  engaged.  It  is  therefore  seen 
that  the  selector  not  only  picks  out  idle  cords,  but  idle  operators  also. 
When  any  operator's  position  is  vacant,  the  switch  17,  corresponding  to 
the  position,  is  thrown  and  this  prevents  the  transfer  of  any  calls  to  it. 

Results  Secured  by  the  Aid-system. — Exhaustive  trials  as  to  the 
working  and  economy  of  the  "Aid-system"  were  made  in  the  central 
office  in  Copenhagen,  called  "Obro."  There  were  fifteen  "A"  operators 
with  90  per  cent  out-trunking.  At  the  beginning  of  these  trials  (spring 
of  1909),  fourteen  of  these  positions  each  had  160  working  subscribers' 
lines.  The  "Aid-system"  allowed  them  in  the  course  of  a  year  to  assign 
220  jacks  to  a  position,  reducing  the  number  of  positions  in  use  from 
fourteen  to  ten. 

Two  other  improvements  are  reported,  as  follows: 

Shorter  waiting  time  for  subscribers.  Vacated  operators'  positions 
now  available  for  growth. 

Part  of  the  economy  secured  was  the  result  of  close  study  of  conditions, 
but  the  "Aid-system"  was  a  large  factor.  It  appealed  to  the  operators 
and  secured  their  hearty  co-operation.  Nevertheless  later  plans  were 
all  made  for  total  traffic  distribution  by  finder  switches. 

The  Stromb erg-Carlson  Traffic  Distributor  Installation. — The  Strom- 
berg-Carlson  Manufacturing  Company  of  Rochester,  N.  Y.,  has  made  two 
installations  of  the  traffic  distributor  type,  one  in  one  of  the  telephone 
offices  in  Rochester,  N.  Y.,  the  other  in  York,  Pa.  The  details  of  these 
installations  are  not  available  for  publication,  but  the  general  principles 
are  these : 

A  number  of  the  busiest  lines  entering  the  office  are  terminated  in 


21G  AUTOMATIC  TELEPHONY 

automatic  lineswitchcs  of  the  rotary  type.  The  trunks  from  these  line- 
switches  are  divided  among  the  "A"  operators.  The  calls  on  these 
busy  lines  are  distributed  among  the  "A"  operators.  By  arranging  it 
so  that  a  lineswitch  will  seek  an  operator  who  is  not  engaged  in  handling 
calls  from  the  lines  permanently  assigned  to  her  and  terminating  in  line 
jacks  in  her  position,  much  can  be  accomplished  in  the  way  of  distribu- 
ing  the  load  and  increasing  the  efficiency  of  the  operators. 

The  Automatic  Electric  Company's  Traffic  Distributor  Equipment. — 
Several  years  ago  the  Automatic  Electric  Company  brought  out  equip- 
ment for  distributing  traffic  more  completely  than  by  the  Christensen 
or  the  Stromberg-Carlson  method. 

In  the  general  scheme  of  the  Automatic  Electric  Company,  all  sub- 
scribers' lines  terminate  in  lines  witches  of  the  Keith  type.  The  trunks  from 
the  lineswitches  terminate  in  cords  and  plugs  in  the  "A  "  operator's  posi- 
tions. If  not  more  than  ten  operators'  positions  are  required,  one  of  the  ten 
trunks  outgoing  from  each  lineswitch  unit  will  lead  to  each  opera- 
tor's position.  In  this  way  the  traffic  from  each  unit  will  be  distributed 
among  all  positions.  If  the  office  should  be  sufficiently  large,  the  scheme 
includes  secondary  lineswitches  in  which  the  trunks  from  the  primary  line 
switchboards  would  terminate  while  the  trunks  from  the  secondary  line- 
switch  groups  would  lead  to  the  operators'  positions. 

From  the  description  of  these  lineswitches,  which  has  been  given  in 
other  chapters,  it  will  readily  be  understood  that  if  a  subscriber  to  an 
office  in  which  the  traffic  distributor  was  installed  should  lift  his  receiver 
from  its  switch-hook,  the  lineswitch  in  which  his  line  terminated  would 
extend  his  line  to  an  idle  cord  circuit.  A  lamp  associated  with  the  cord 
circuit  would  immediately  light,  the  operator  would  throw  her  listening 
key,  take  the  subscriber's  number  and  complete  the  connection  by  plug- 
ging into  the  multiple  in  the  customary  way;  or,  if  the  subscriber  desired 
a  party  connected  to  another  office,  she  would  press  her  order-wire  key 
and  secure  a  trunk  to  the  distant  office  from  the  proper  " B"  operator,  as 
in  regular  manual  practice. 

The  plan  embodies  circuit  arrangements  which  make  it  possible  for 
any  operator  to  leave  her  position,  after  first  making  busy  the  trunks 
terminating  in  it,  by  throwing  keys  associated  with  those  trunks.  During 
less  busy  hours,  the  work  to  be  done  is  placed  immediately  in  front  of  the 
operators  remaining  at  the  switchboard  so  that  they  never  have  to  reach 
over  the  positions  on  either  side  of  them.  This  feature  is  intended  to 
eliminate,  so  far  as  is  possible,  the  loss  of  efficiency  due  to  the  first  dif- 
ficulty mentioned  in  the  opening  paragraphs  of  this  chapter. 

The  inventors  have  worked  out  theoretically  the  efficiencies  which 
may  be  expected  to  be  secured  by  its  use.  Their  deductions  have  been 
very  closely  approximated  by  the  actual  performance  of  several  plants 
which  have  been  in  operation  from  four  to  six  years.     These  theoretical 


AUTOMATIC  TRAFFIC  DISTRIBUTOR  EQUIPMENT 


217 


efficiencies  are  presented  here  as  a  quotation  from  the  first  edition  of  this 
book. 

"It  is  expected  that  the  merging  of  all  traffic  into  one  large  group  will  (as 
far  as  possible)  even  up  the  load  between  positions  and  distribute  momentary 
fluctuations.  Greater  efficiency  certainly  should  result,  because  of  proportion- 
ing the  work  to  each  operator's  ability  and  offering  rewards  for  high  efficiency. 

"It  has  been  calculated  that  the  omission  of  the  labor  of  inserting  and  later 
removing  the  answering  plug  will  reduce  the  operator's  work  22  per  cent  (single 
office,  individual  line,  flat  rate).  It  is  expected  to  increase  the  actual  busy- 
hour  working  time  from  two-thirds  to  five-sixths  of  the  hour.  This  will  make 
a  total  increase  in  efficiency  of  60.6  per  cent  in  the  busy  hour,  requiring  38  per 
cent  less  operators  than  usual. 

"Practice  shows  that  an  operator's  average  load  is  about  75  per  cent  of  her 
busy-hour  load.  If  traffic  distribution  will  enable  the  busy-hour  load  to  be 
maintained,  it  will  add  33  H  per  cent  to  the  efficiency.  This  reduces  the  operator- 
hours  for  the  day  to  about  47  per  cent  of  that  required  without  distribution. 

"Fewer  busy-hour  operators  means  a  smaller  switchboard,  with  saving  in 
multiple  sections,  jacks,  cable  and  equipment. 


Cord  Circuit 


Open 
flo  in 
(-Battery)  5£ 

Fig.   194. — Automatic  traffic  distributor,  lineswitch  and  cord  circuit. 


"Of  course  the  efficiency  estimated  above  could  not  be  realized  where  a 
large  percentage  of  calls  is  trunked  to  other  offices,  or  where  the  operators  work 
is  slowed  down  by  measured  service  conditions,  etc.,  but  doubtless  the  saving 
would  still  be  a  great  one." 

Typical  Traffic  Distributor  Circuits  Using  Keith  Lineswitch  Equip- 
ment.— In  Fig.  194  is  shown  the  circuit  of  a  simple  traffic  distributor 
arrangement  using  primary  Keith  lineswitches.  The  lineswitch  has 
associated  with  it  a  multiple  jack,  which  is  mounted  in  the  manual 
switchboard.  Each  subscriber's  line  terminates  in  these  two  pieces  of 
apparatus.  The  telephones  used  with  this  equipment  may  be  of  any 
regular  common-battery  manual  type. 

When  a  calling  subscriber  removes  his  receiver  from  the  switch-hook, 
the  lineswitch  operates  in  the  usual  manner  and  extends  the  line  to  a 
trunk.  The  grounding  of  the  same  bank  contact  which  moves  the  master 
switch  also  energizes  the  chain  relay  C.R.  whose  function  has  been  fully 
described.     Current  is   now  supplied  to  the   calling  subscriber's  loop 


218  AUTOMATIC  TELEPHONY 

through  the  windings  of  the  answering  bridge-relay  (A.B.R.)  of  the  cord 
circuit.  A.B.R.  closes  circuit  from  earth  through  the  release-trunk  R.T., 
the  primary  lineswitch  bank  contacts,  and  the  bridge  cut-off  winding  of 
the  lineswitch  to  negative  battery.  The  completion  of  this  circuit 
causes  the  lineswitch  to  hold  its  plunger  in  the  bank.  At  the  same  time 
A  .B.R.  closes  circuit  through  the  answering  signal  lamp  A .  The  operator 
responds  by  throwing  the  listening  key  of  the  cord  circuit  corresponding 
to  the  glowing  lamp,  thus  breaking  the  circuit  through  A  and  connecting 
her  operator's  set  across  the  line.  She  takes  the  subscriber's  number  in 
the  manner  customary  in  regular  manual  practice,  then  picks  up  the 
plug  of  the  cord  circuit  and  touches  its  tip  to  the  sleeve  of  the  jack  of  the 
desired  subscriber's  line.  If  this  line  is  busy,  the  sleeve  will  be  connected 
to  earth,  because  it  is  connected  to  the  release  trunk  of  the  lineswitch. 
Therefore,  a  circuit  will  be  completed  from  earth  through  the  tip  of  the 
plug,  contacts  of  the  sleeve  relay,  S.R.,  and  one  winding  of  the  operator's 
induction  coil  to  negative  battery,  giving  her  the  customary  busy  click. 
If  the  line  is  not  engaged,  she  inserts  the  plug  into  the  jack,  whereupon 
current  immediately  flows  from  earth  through  the  winding  of  the  sleeve 
relay,  sleeve  of  the  plug,  sleeve  of  the  jack  and  the  bridge  cut-off  winding 
of  the  called  party's  lineswitch  to  negative  battery.  When  that  bridge 
cut-off  relay  armature  operates,  it  disconnects  the  line  relay  winding  and 
extends  the  connection  through  to  the  called  party's  telephone,  as  in  full 
automatic  practice.  At  the  same  time  the  sleeve  relay  connects  the 
release  trunk  to  earth,  preventing  the  lineswitch  from  being  released 
until  the  operator  disconnects.  This  relay  also  closes  circuit  from  earth 
through  the  calling  signal  lamp  C,  which  remains  lighted  until  the  called 
party  responds,  when  its  circuit  is  opened  by  the  calling  bridge  relay 
C.B.R.,  through  whose  windings  talking  current  is  supplied  to  the  called 
party. 

The  called  party  is  rung  manually  by  means  of  a  ringing  key. 

When  the  calling  party  replaces  his  receiver  on  the  switch-hook,  lamp 
A  again  lights;  and  when  the  called  party  hangs  up,  lamp  C  lights. 
Either  party  may  flash  the  lamp  belonging  to  his  end  of  the  circuit  at 
any  time.  When  conversation  is  completed  and  the  operator  withdraws 
the  plug  from  the  jack,  the  lineswitch  releases. 

At  any  time  the  operator  wishes  to  make  any  particular  cord  circuit 
busy,  because  it  is  in  need  of  repairs,  she  may  do  so  by  throwing  the 
"make  busy"  key.  When  she  leaves  her  position  she  must  throw  the 
"make  busy"  key  of  each  cord  circuit  terminating  in  it,  thus  causing  all 
calls,  which  might  have  come  to  her  position,  to  be  automatically  dis- 
tributed among  the  remaining  positions.  Since,  if  lineswitch  plungers 
of  the  ordinary  type  should  be  used,  some  plunger  poised  over  a  trunk 
leading  to  this  operator's  position  might  be  out  of  engagement  with  the 
master  switch  shaft,  at  the  time  she  leaves  her  position,  it  is  necessary 


AUTOMATIC  TRAFFIC  DISTRIBUTOR  EQUIPMENT  21!) 

to  follow  either  one  of  two  practices  to  prevent  a  calling  party  being 
trunked  to  her  position  while  she  is  absent. 

One  practice  is  to  install  a  key  in  the  chief  operator's  desk,  which  the 
chief  operator  presses  for  an  instant  at  any  time  an  operator  leaves  her 
position,  and  which  causes  all  of  the  master  switches  to  swing  their 
master  shafts  through  their  complete  arc,  thus  picking  up  any  plungers 
which  are  out  of  engagement. 

Another  practice  is  to  use  what  is  called  a  self-restoring  type  of 
plunger.  The  construction  of  this  plunger  is  such  that  when  it  has  with 
drawn  from  the  bank  it  instantly  lines  up  with  the  other  idle  plungers. 

In  a  traffic  distributor  arrangement  for  a  much  larger  office,  secondary 
lineswitches  are  required  on  account  of  the  large  number  of  operators' 
positions,  and  where  all  service  is  measured  so  that  each  call  must  be 
registered,  by  the  operator  handling  it,  a  meter  is  associated  with  each 
subscriber's  line. 

Keith  Lineswitches  Used  on  Order  Wires  Between  "A"  and  "B" 
O  erators. — A  circuit  for  use  in  distributing  the  load  among  "B"  oper- 
ators, who  receive  their  orders  over  order  wires  from1  "A"  operators,  is 
shown  in  Fig.  195. 

The  lineswitch  used  is  of  the  "secondary"  type,  because  no  bridge 
cut-off  relay  is  required.  The  switch  is  operated  and  kept  in  operated 
condition  during  the  conversation  between  an  "A"  and  "B"  operator, 
by  means  of  the  holding  trunk  wire,  which  is  connected  to  earth  when  the 
"A"  operator  depresses  her  order-wire  key.  This  system  is  intended 
for  use  only  between  branch  offices  of  considerable  size.  Each  "A" 
operator's  position  would  be  equipped  with  one  order-wire  key  only  for 
each  other  office  unless  it  were  thought  that  a  second  key  was  necessary 
for  use  in  case  the  first  key  or  some  of  the  circuits  or  apparatus  connected 
with  it  should  require  repairs. 

The  circuits  of  the  operators'  sets  and  of  the  order-wire  key  are  not 
shown,  because  they  might  be  of  any  ordinary  design.  The  only  depart- 
ure from  regular  practice  would  be  that  already  mentioned,  that  is  to 
arrange  the  key  so  that  it  would  connect  the  holding  trunk  to  earth. 

The  operation  of  this  system  is  as  follows: 

An  operator  desiring  to  transmit  an  order  to  a  "B"  operator  will  de- 
press the  order-wire  key  for  the  proper  office,  whereupon  the  holding  trunk 
will  be  connected  to  earth  and  the  lineswitch  will  thrust  its  plunger  into 
bank  contacts  corresponding  to  a  trunk  to  an  idle  "B"  operator.  The 
"A  "  operator's  talking  set  will  be  connected  to  the  order  wire  at  the  same 
time,  and  without  waiting  a  response  from  the  " B"  operator  she  will 
immediately  order  up  the  connection  desired,  whereupon  the  "B" 
operator  will  assign  an  idle  trunk  in  the  usual  manner.  When  the  "A" 
operator  removes  her  finger  from  the  order-wire  key,  the  lineswitch 
restores  to  normal  position. 


220 


AUTOMATIC  TELEPHONY 


A  "B"  operator's  position  may  be  made  busy  to  all  incoming  calls 
when  desired  by  throwing  the  order-wire  busy  key  shown  in  the  diagram, 
thereby  operating  the  1300-ohm  relay  bridged  across  the  circuit.  The 
effect  is  to  connect  to  earth  the  master  switch  bank  contact  corresponding 


fi^yt  &JIM  J3pjQ  ox 


to  the  trunk.  At  any  time  when  the  "B"  operators  of  a  certain  group 
are  all  busy,  all  the  chain  relays  of  that  group  will  be  energized,  the  circuit 
of  the  stop  relay  will  be  closed  and  the  busy  circuit  will  be  closed  through 
the  primary  of  the  busy  induction  coil.     Since  the  negative  battery  con- 


AUTOMATIC  TRAFFIC  DISTRIBUTOR  EQUIPMENT 


221 


nection  to  all  lineswitch  pull-down  coils  is  cut  off  when  the  stop  relay  is 
energized,  no  lineswitch  can  be  operated  under  these  conditions,  and  the 
busy  signal  is  transmitted  through  the  back  contacts  of  the  lineswitch 
relay  to  any  "A"  operator  depressing  an  order-wire  key.  The  instant 
any  chain  relay  is  released,  however,  the  lineswitch  of  the  operator  who 
has  depressed  her  order-wire  key  seeking  a  connection  will  seize  the  idle 
order- wire  trunk.  The  busy  signal  is  closed  during  the  operation  of  the 
master  switch  mechanism  also. 

The  apparatus  may  be  used  either  with,  or  without,  what  is  called  the 
position  blocking  feature,  as  desired.  That  is,  it  may  be  arranged  to  dis- 
tribute the  load  evenly  among  the  "B"  operators,  regardless  of  the  rela- 


ftefay  T  operative  before  Relay  Hh  the  Meter\ 

I  a  tier  being  slow- acting  due  tons  150  Ohm 


1°°„  winding  being  short 
circuited-* 


•.    ^P/<5    B-Posith 


Fig.  196.- 


Position  6  to last  Po. 

To  Pilot  Control 
.      Key  Postn4 

rc=~  These  Keys  are  thrown  .  V=^-? "  &  & 
^  ■?  J_  when  corresponding  position  _|_  g-a 
^>     <     ~T~  /s  occupied     — ]-^>-4 

^£       ---\BusyPlfotLamp  \ 

+  on  center  Panel  \    j; 

/  per  Position  § 

*  These  Keys  are  thrown  when  correspond- 
ing Position  is  occupied/ 

-Circuit  of  Siemens-Halske  call  wire  selector  installed  at  the  central  exchange 
in  London. 


To  Interrupter 


tive  efficiencies  of  the  operators.  With  this  arrangement  two  or  more 
"A"  operators  might  secure  connection  to  a  given  "Z?"  operator  simul- 
taneously, as  in  straight  manual  practice,  but  on  the  whole  the  work 
would  be  evenly  distributed  among  the  " B"  operators. 

Since  with  the  blocking  feature  any  "B"  operator's  position  is  Dusy, 
however,  for  such  a  very  short  period  of  time,  that  is,  during  the  time  only 
that  an  "A"  operator's  order-wire  key  is  depressed,  and  since  no  appre- 
ciable period  of  time  need  elapse  between  the  release  of  a  trunk  by  one 
lineswitch  and  the  seizure  of  it  by  another,  it  would  appear  that  the 
position-blocking  arrangement  would  not  only  be  the  most  efficient  and 
systematic  one  generally,  but  that  even  at  rush  moments  it  will  handle  the 
traffic  as  well  as  the  other  plan,  because  no  matter  how  great  the  need 


222  AUTOMATIC  TELEPHONY 

for  her  services  may  be,  a  "B"  operator  cannot  take  two  orders  and  put 
up  two  connections  at  the  same  time. 

Siemens-Halske  Co's  Traffic  Distributor  Equipment. — An  automatic 
call-wire  selecting  equipment  installed  by  Siemens  Brothers  &  Company 
Limited,  and  of  the  Siemens-Halske  Company's  design  has  recently  been 
put  into  service  on  trial  in  the  Central  Exchange,  London.  The  following 
description  of  it  is  quoted  from  a  paper  presented  by  Mr.  W.  Slingo  before 
"The  Institution  of  Electrical  Engineers." 

An  automatic  call-wire  selecting  equipment  has  been  installed  at  the  Central 
Exchange,  London,  which  has  for  its  object  the  selection  and  isolation  of  a  call- 
wire  when  one  is  required  by  an  "A"  operator.  There  are  ten  call-wires  in 
the  group  affected,  and  they  were,  before  the  introduction  of  the  new  condi- 
tions, operated  from  ten  call-wire  keys,  the  circuits  of  which  were  multiplied 
over  160  "A"  positions.  When  the  selecting  equipment  was  introduced  the 
ten  keys  per  position  were  replaced  by  one  key  per  position. 

Each  operator  is  given  a  selector  which  rotates  when  she  depresses  her  call- 
wire  key.  The  rotation  continues  until  the  wipers  find  an  idle  "B"  operator, 
when  the  "A"  operator  is  able  to  pass  her  call.  Figure  196  gives  the  circuit 
arrangement. 

DETAILS  OF  SELECTIVE  MECHANISM 

Each  "A  "  position  is  provided  with  a  selector,  and  each  selector  has  associated 
with  it  two  relays,  an  "R"  relay  and  a  "T"  relay.  The  contact  banks  of  the 
selectors  are  multiplied  together  and  are  connected  to  the  group  of  "£"  positions, 
so  that  each  "B"  position  is  represented  on  each  bank  by  a  separate  set  of 
contacts.  The  "B"  positions  have  also  each  associated  with  them  an  "H" 
relay  and  a  retardation  coil. 

The  "a"  and  "6"  arms  of  the  selectors  are  connected  with  the  "A"  operator's 
telephone  circuits  through  contacts  of  the  associated  "  7"'  relays,  and  the  selector 
bank  contacts  are  connected  with  the  "B"  operator's  telephone  circuits  through 
contacts  of  the  "H"  relays. 

On  the  depression  of  a  call-wire  key  by  an  "A"  operator,  the  arms  of  that 
selector  which  corresponds  to  the  particular  position  concerned  commence  to  seek 
over  the  contact  bank,  and  automatically  come  to  rest  on  the  contacts  of  the  first 
accessible  and  disengaged  "B"  position.  The  "J1"  relay  associated  with  the 
*'A"  position  and  the  "H"  relay  associated  with  the  "B"  position  are  now 
operated,  so  that  the  two  sperking  circuits  are  connected. 

On  the  release  of  the  call- wire  key  on  the  "  A  "  position  the  selector  arms  do  not 
move  from  the  position  occupied.  The  "T"  and  "H"  relays  are,  however, 
released,  and  disconnect  the  "A"  operator's  telephone  circuit  from  the  "B" 
operator's  circuit. 

During  the  time  the  call-wire  is  in  use  the  "  T"  relay  remains  operated,  and 
this  renders  the  multiple  contacts,  on  the  particular  "B"  position,  engaged 
against  the  other  selectors  by  reducing  the  potential  on  the  "t"  contacts  from  22 
volts  to  nearly  earth  potential.  The  "J"'  relays  of  other  selectors  whose  arms 
may  pass  over  these  contacts  therefore  will  not  be  operated. 

When  a  "B"  position  is  rendered  inaccessible  to  the  selectors  by  the  "B" 


AUTOMATIC  TRAFFIC  DISTRIBUTOR  EQUIPMENT  223 


It  4  •' 


position  call-wire  key  being  normal,  the  battery  is  disconnected  from  the  "V 
contacts  corresponding  to  the  position,  so  that  in  this  case  also  the  "T"  relays  of 
searching  selectors  will  not  be  operated. 

Motor  interrupters  are  used  for  supplying  the  interrupted  current  to  drive  the 
selectors.  These  are  provided  in  duplicate,  and  there  are  also  two  separate  sets  of 
supply  mains  available  for  driving  the  motors. 

The  interrupters  themselves  have  each  20  sets  of  contact  springs  (ten  of 
which  are  at  present  spare).  Each  spring  set  supplies  driving  current  to  20 
selectors,  these  being  distributed  between  four  time-fuse  mountings,  so  that  only 
alternate  positions  at  the  switchboard  are  supplied  through  the  same  fuse  and 
spring  set. 

In  order  to  absorb  the  spark  which  would  otherwise  occur  at  the  contacts  of 
the  interrupter  springs,  a  circuit  consisting  of  a  0.5-ohm  resistance  coil  and  a 
4  m.f.  condenser  in  series  is  bridged  across  each  pair  of  springs. 

The  current  supplied  to  the  interrupter  spring  sets  is  taken  from  a  30-volt 

supply. 

Operation  of  Circuits. — On  the  depression  of  a  call-wire  key  by  an  "A" 
operator  a  circuit  is  closed  through  relay  R. 

Relay  R  operating  contact  r  is  arranged  so  that  the  spring  x  makes  contact 
with  the  spring  r  before  contact  is  made  with  the  spring  y ;  r  therefore  first  pre- 
pares the  circuit  of  the  testing  relay  T  from  earth  through  T  300  and  10  to  the 
"t "  arm  of  the  selector;  and  second,  completes  the  circuit  of  the  driving  magnet. 

The  other  contact  of  relay  R  is  used  to  complete  the  circuit  of  the  night-bell 
relay  and  to  start  the  motor  interrupter  during  the  period  of  night  working. 

The  circuit  of  the  selector-driving  magnet  now  being  completed,  the  selector 
arms  "a,"  "b,"  and  "t"  are  driven  over  the  bank  of  contacts  representing  the 
"B"  positions,  and  when  a  set  of  contacts  representing  a  free  "B"  operator  is 
found,  the  circuit  of  relays  T  (300  and  10-ohm  coils  in  series)  and  H\  (50-ohm 
coil  only)  is  completed  through  the  arm  "t"  and  a  "B"  position  call-wire  key. 
Relays  T  and  Hi  are  now  operated,  but  relay  Hh  since  its  150-ohm  coil  is 
short-circuited  by  one  of  its  own  contacts,  is  slow  acting,  and  relay  T  is  thus 
allowed  to  operate  slightly  before  it. 

The  line  contacts  of  relay  T  connect  the  "A"  operator's  instrument  circuit  to 
the  selector  arms  "a"  and  "6,"  and  current  then  flows  through  the  2500  +  2500 
ohm  coil  and  the  "A"  .operator's  instrument  circuit.  A  click  is  given  in  the 
"A"  operator's  receiver,  denoting  that  a  free  "B"  operator  has  been  found,  and 
the  potential  difference  between  the  arms  "a"  and  "b"  falls  to  a  small  fraction 
of  22  volts. 

Another  contact  of  relay  T  disconnects  the  circuit  of  the  driving  magnet, 
preventing  further  movement  on  the  part  of  the  selector  arms,  and  also  engages 
the  "B"  position  against  other  selectors  by  short-circuiting  the  300-ohm  coil  of 
relay  T.  Relay  T  now  holds  through  its  10-ohm  coil,  and  the  "T"  relay  of  any 
other  selector  testing  on  a  multiple  of  this  contact  will  have  its  300-ohm  coil 
shunted  by  the  10-ohm  coil  of  the  engaging  relay  T,  and  will  consequently  not 
receive  sufficient  current  to  operate. 

The  line  contacts  of  relay  Hx  close,  completing  the  circuit  of  the  speaking  leads 
between  the  "A"  operator's  and  the  "B"  operator's  instruments.  No  appre- 
ciable  click  is  produced  in  the  "B"  operator's  receiver  when  these  contacts 


224  AUTOMATIC  TELEPHONY 

close,  as  the  potential  difference  existing  between  the  "a"  and  "b"  wires  is  by 
this  time  reduced  to  a  small  value  (see  above). 

Of  the  further  three  contacts  of  relay  Hi  one  contact  closes  and  operates  the 
meter  associated  with  the  "B"  position,  registering  one  call;  another  removes 
the  short-circuit  from  the  150-ohm  coil  of  relay  Hi',  and  the  remaining  contact 
completes  the  portion  of  the  circuit  of  the  chief  supervisor's  lamp  which  belongs 
to  the  particular  "B"  position. 

The  "A"  operator  will  now  pass  the  call  to  the  "B"  operator,  and  will  be 
assigned  a  junction  in  the  usual  manner;  she  will  then  release  the  call-wire  key 
at  the  "A  "  position  and  disconnect  the  circuit  of  relay  R. 

Relay  R  releases. 

Contact  r  disconnects  the  circuit  through  the  relays  T  and  Hi,  and  here  again 
it  will  be  noted  that  the  circuit  of  the  driving  magnet  is  opened  at  contacts  x 
and  y  before  the  circuit  of  the  "T"  relay  is  disconnected,  thus  preventing  any 
possibility  of  the  selector  stepping  forward  during  the  period  of  release. 

Relays  T  and  Hi  release. 

Of  relay  T  the  line  contacts  disconnect  the  "A"  operator's  telephone  circuit 
from  the  selector  arms,  thus  preventing  interference  with  other  circuits  when 
the  arms  commence  to  seek. 

Of  the  other  two  contacts  of  relay  T  one  prepares  the  circuit  of  the  driving 
magnet  for  further  use,  and  the  other  prepares  the  night-bell  circuit  and  the 
motor-starting  circuit. 

Of  relay  Hi  the  line  contacts  disconnect  the  "B"  operator's  telephone  circuit 
from  the  contact  banks  of  the  selector  and  open  the  circuit  through  the  retard  coil 
K  and  the  "B"  operator's  telephone. 

Of  the  further  three  contacts  of  relay  Hi,  one  releases  the  meter  associated 
with  the  "5"  position,  another  short-circuits  the  150-ohm  coil  of  the  relay  Hi, 
thus  making  it  again  slow-acting;  and  the  remaining  contact  opens  the  circuit 
of  the  chief  supervisor's  lamp  at  this  position. 

When  it  is  desired  to  render  a  "B"  position  inaccessible  to  the  "A"  position 
selectors,  the  call- wire  circuit  key  on  the  "B"  position  is  returned  to  normal. 
This  disconnects  the  contacts  on  the  "t"  banks  of  the  selector  multiple  from  the 
"H"  relay  corresponding  to  that  position.  The  "T"  relays  of  selectors  whose 
arms  may  seek  over  these  contacts  will  therefore  not  be  operated. 

When  the  call- wire  circuit  key  on  a  "  B"  position  is  normal  a  circuit  is  closed 
through:  Positive,  inside  contact  of  call- wire  circuit  key,  contact  of  pilot  control 
key,  busy  pilot  lamp,  —22  volts.  This  lights  the  busy  pilot  lamp  situated  above 
the  position.  Should  it  not  be  necessary  for  this  lamp  to  remain  lighted  its  circuit 
can  be  disconnected  by  means  of  the  pilot-control  key  and  the  lamp  extinguished. 
When  the  call-wire  circuit  key  is  not  thrown  another  circuit  is  closed  through: 
Positive,  outside  contact  of  call-wire  circuit  key,  back  contact  of  pilot-control  key 
lamp,  — 22  volts,  and  the  lamp  will  again  light. 

The  circuit  for  the  chief  supervisor's  lamp  is  completed  when  all  the  "B" 
positions  which  have  not  been  rendered  inaccessible  to  the  selectors  are  simul- 
taneously engaged.  The  circuit  will  then  be  from  —22  volts,  through  the  pilot 
relay  associated  with  the  desk,  the  chief  supervisor's  lamp,  contact  of  the  special 
night-bell  key,  the  contacts  of  relay  H  on  those  positions  which  are  engaged, 
and  the  call-wire  key  contacts  of  those  positions  which  are  inaccessible  to  the 
selectors,  to  positive. 


AUTOMATIC  TRAFFIC  DISTRIBUTOR  EQUIPMENT  225 

If  an  "A"  position  selector  develops  a  mechanical  fault  a  spare  selector  can  be 
connected  up  to  the  "A  "  position  affected  as  follows: 

Take  out  the  "  R"  relay  fuse  of  the  faulty  "A"  position,  insert  the  "R"  relay 
fuse  of  the  spare  selector  and  connect  a,  b,  and  t  terminals  of  the  two  selectors 
together  on  the  terminal  strips  above  the  selector  racks.  This  procedure  applies 
to  other  than  mechanical  faults  on  a  selector,  as  is  obvious  from  an  inspection 
of  Fig.  196. 

If  the  supply  to  both  interrupter  motors  fails,  the  selectors  must  be  adjusted 
by  hand  to  distribute  the  traffic  among  the  "B"  positions  according  to  the 
requirements  of  the  exchange  manager.  The  ordinary  method  of  call-wire 
working  will  then  apply. 


15 


CHAPTER  IX 

AUTOMATIC     SUB-OFFICES    IN     CONNECTION     WITH 
MANUAL  CENTRAL  OFFICES 

The  purpose  of  an  automatic  sub-office  is  to  save  cable  and  conduit 
between  a  district  and  the  central  office.  Sometimes  the  outgrown  line 
cable  will  serve  as  a  trunk  cable  to  the  sub-office. 

The  same  reasons  which  make  sub-offices  desirable  in  connection  with 
automatic  central  offices  create  a  demand  for  them  in  connection  with 
manual  central  offices.  These  reasons  are  satisfied  to  better  advantage 
with  an  automatic  sub-office  than  with  a  branch,  manually  operated 
switch  board,  because,  while  the  latter  may  be  less  expensive  to  install, 
it  increases  the  operating  cost,  slows  up  the  service,  increases  the  chances 
for  wrong  connections,  premature  disconnections,  and  other  troubles 
which  are  inherent  to  the  setting  up  of  connections  where  both  "A  "  and 
" B"  operators  are  required. 

When  an  automatic  sub-office  is  used,  the  calls  from  the  branch 
office  locality  to  the  main  office  are  handled  as  speedly  as  if  the  lines  were 
connected  directly  to  the  main  office.  Calls  going  in  the  opposite  direc- 
tion preferably  pass  through  two  operators,  but  the  second  operator  is 
located  at  the  main  office  so  that  at  nights  and  on  Sundays  economy  may 
be  practised  by  having  one  of  the  few  operators  on  duty  attend  to  the 
work  at  this  "E"  position  also.  With  a  branch  manual  switchboard  it  is 
always  necessary  to  have  some  one  on  duty  at  all  hours  of  the  day  and 
night. 

There  are  other  reasons  which  make  sub-offices  in  connection  with 
manual  plants  attractive  under  certain  conditions.  Sometimes  a  company 
has  outgrown  its  multiple  switchboard,  or  has  reached  a  point  in  the 
growth  of  the  switchboard  where  each  additional  section  added  to  it  is 
very  expensive,  because  the  multiple  must  be  increased  throughout  the 
entire  board.  This  expense  can  be  saved  by  putting  one  or  more  sub- 
offices  in  some  of  the  out-lying  localities  and  reserving  the  line  jacks  and 
multiples  of  the  main-office  switchboard  for  the  shorter  lines.  Sometimes 
the  company's  equipment  has  grown  to  the  limit  of  its  switchboard  room, 
so  that  to  enlarge  it  will  require  an  addition  to  the  building.  This  dif- 
ficulty may  also  be  met  by  the  use  of  an  automatic  sub-office. 

Again,  a  company  may  be  expecting  to  change  to  some  form  of  auto- 
matic equipment  in  a  comparatively  few  years  and  may  have  decided 
which  type  of  automatic  equipment  it  will  use  and  will,  therefore,  not 

226 


AUTOMATIC  SUB-OFFICES  227 

wish  to  invest  any  more  money  than  necessary  in  additions  to  its  manual 
switchboard.  Such  additions  may  be  avoided  to  a  large  extent,  in  cities 
large  enough  to  warrant  the  use  of  sub-offices,  by  installing  one  or  more  of 
them  to  take  care  of  the  required  growth;  and  these  offices  will  be  available, 
with  slight  changes,  for  use  with  an  automatic  central  office  when  it  is 
installed. 

The  average  manual  switchboard  may  be  used  with  a  large  number  of 
different  sub-office  arrangements,  and  such  offices  have  been  installed 
using  the  equipment  manufactured  by  the  Automatic  Electric  Company, 
the  American  Automatic  Telephone  Company,  and  the  Siemens-Halske 
Company  of  Germany;  but  in  order  to  make  this  chapter  clear  and 
concise,  a  limited  number  of  typical  arrangements  will  be  outlined,  and 
before  explaining  the  details  of  circuits,  several  general  plans  will  be 
discussed. 

General  Plans. — The  first  of  these  plans,  which  will  be  called  Plan  No. 
1,  requires  no  appreciable  change  in  any  of  the  circuits  or  equipment  of 
almost  any  ordinary  common-battery  multiple  switchboard,  and  there- 
fore may  be  installed  in  connection  with  almost  any  central  office  without 
large  expense  for  changing  or  adding  to  the  central-office  equipment. 

Plan  No.  2  requires  more  changes  than  Plan  No.  1  in  the  central- 
office  switchboard,  but  is  the  easier  and  more  economical  of  operation. 

PLAN  NO.  1 

Incoming  Calls. — In  this  plan  the  trunks  incoming  to  main  office  from 
the  banks  of  the  lineswitches  or  from  finder  switches  installed  at  the  sub- 
office  terminate  at  the  central  office  in  the  regular  subscribers'  line  equip- 
ments of  the  multiple  switchboard.  The  lines  may  be  divided  among 
the  various  "A"  operators,  or  may  be  assigned  to  one  operator.  The 
former  is  the  preferable  plan  when  a  comparatively  small  number  of 
calls  is  made  between  sub-office  subscribers;  but  where  the  percentage 
of  local  interconnections  is  large,  it  is  better  to  have  the  incoming  trunks 
go  to  one  operator;  and,  preferably,  to  the  one  who  also  handles  the  out- 
going calls  to  the  sub-office,  so  that  the  interconnections  may  be  made 
with  the  least  effort. 

If  a  sub-office  subscriber  desires  a  connection  to  a  main-office  sub- 
scriber, he  removes  his  receiver  from  the  switch-hook  in  the  usual  manner 
and  places  it  to  his  ear,  awaiting  the  answer  of  the  operator.  The  instant 
the  switch-hook  rises,  the  subscriber's  lineswitch  or  an  idle  trunk-finder 
switch  (of  whatever  type  or  manufacture  it  may  be)  places  his  line  in  con- 
nection with  an  idle  trunk  with  the  result  that  the  line  lamp  corresponding 
to  the  trunk  selected  glows  in  front  of  the  central-office  operator  before 
whom  the  trunk  terminates.  She  plugs  in,  takes  the  subscriber's  order, 
and  completes  the  connection  by  plugging  in  to  the  desired  party's 


228  AUTOMATIC  TELEPHONY 

multiple  jack  and  ringing  in  the  usual  manner.  When  the  subscribers 
finish  talking  they  hang  up  their  receivers,  whereupon  the  operator  re- 
ceives the  customary  supervisory  signals  and  pulls  down  the  connection. 
It  may  be  arranged  so  that  the  calling  subscriber's  lineswitch  will  release 
when  he  hangs  up,  or  so  that  the  trunk  will  be  kept  busy  until  the  operator 
pulls  down  the  cords,  thus  preventing  any  possibility  of  another  sub- 
scriber being  switched  to  the  same  trunk  after  the  first  subscriber  has 
disconnected  and  before  the  operator  has  pulled  down  her  cords.  Should 
a  subscriber,  who  has  a  connection,  desire  to  attract  the  operator's  atten- 
tion at  any  time,  he  can  do  so  in  the  usual  manner  by  moving  his  switch- 
hook  up  and  down,  thus  flashing  the  operator's  supervisory  lamp.  The 
talking  current  for  the  sub-office  may  be  fed  from  the  cord  circuit  at 
the  main  office  or  from  the  sub-office  battery,  as  desired;  generally  the 
latter  is  preferable  from  the  standpoint  of  transmission  efficiency. 

Outgoing  Calls. — These  calls  are  put  up  by  the  use  of  a  calling  device, 
at  the  manual  central  office,  by  means  of  which  connector  switches  in- 
stalled at  the  sub-office  are  operated  over  the  out  going  trunks  frcm  the 
central  office  to  the  sub-office.  A  calling  device  may  be  made  available 
to  each  "A  "  operator,  enabling  her  to  complete  connections  from  a  cen- 
tral office  subscriber  to  a  sub-office  subscriber  directly,  without  the  aid  of 
a  "B"  operator,  or  the  plan  may  be  such  that  all  out  trunk-connections 
will  be  set  up  by  a  " B"  operator,  at  whose  position  alone  calling  devices 
are  installed.  The  former  plan  may  be  made  practicable  where  the 
percentage  of  calls  to  sub-offices  is  large.  Generally,  however,  the  latter 
method  is  preferable  because  of  the  comparatively  small  number  of  calls 
from  central-office  lines  to  sub-offices  and  the  difficulty  of  training  "A" 
operators,  who  are  accustomed  to  go  through  a  regular,  set  routine  on 
most  of  their  calls,  to  go  quickly  and  accurately  through  a  different 
routine  on  a  call  to  the  sub-office.  If  all  these  calls  pass  through  one  or 
more  operators,  who  are  especially  trained  to  handle  them,  the  efficiency 
of  those  operators  may  be  made  quite  high. 

When  a  "B"  operator  is  used  for  handling  the  outgoing  trunk  calls 
under  Plan  No.  1,  her  position  is  equipped  with  cord  circuits.  When  she 
receives  an  order  from  an  "A"  operator  over  an  order  wire  in  the  usual 
way,  she  picks  up  the  answering  plug  of  an  idle-cord  circuit  and  inserts 
it  into  the  multiple  jack  of  the  calling  subscriber's  line,  and  it  is  therefore 
unnecessary  for  her  to  assign  any  trunk  to  the  "A  "  operator.  The  "A" 
operator's  work  is  now  completed.  The  trunk  operator  at  the  same  time 
picks  up  the  calling  plug  of  the  idle-cord  circuit  used  and  inserts  it  in  the 
jack  of  an  outgoing  trunk  which  terminates  in  an  automatic  connector 
switch  in  the  proper  group  at  the  sub-office.  She  then  throws  the  calling- 
device  key  of  the  cord  circuit  and  makes  two  motions  of  her  calling-device 
dial  to  operate  the  switch  and  complete  the  connection.  The  ringing  of 
the  called  party's  bell  may  be  done  by  the  trunk  operator,  or  may  be 


AUTOMATIC  SUB-OFFICES  229 

done  automatically  by  the  connector  switch,  as  desired.  In  either  event 
the  relays  and  lamps  in  the  "B"  operator's  cord  circuit  give  her  the 
customary  supervisory  signals.  When  the  subscribers  finish  talking  and 
place  their  receivers  on  their  switch-hooks,  the  operator  is  given  the 
signal  and  pulls  down  the  cords.  When  she  removes  the  plug  from  the 
outgoing  trunk  jack  the  connector  switch  used  releases. 

If  the  switchboard  is  equipped  with  trunks  ending  in  jacks  in  the  "  A  " 
operators'  sections  and  in  plugs  in  the  "B"  operators'  position,  it  is  better 
practice  than  that  outlined  in  the  foregoing  paragraph  to  have  the  "B" 
operator  assign  an  idle  trunk  when  the  subscriber's  order  is  received  from 
the  "A"  operator.  The  "A"  operator  then  inserts  the  calling  plug  of 
the  cord  on  which  the  subscriber  is  waiting  in  the  jack  of  the  assigned 
trunk  while  the  "B"  operator  inserts  the  plug  of  the  cord  terminating 
the  transfer  trunk  into  the  jack  of  an  idle,  outgoing  trunk  to  the  sub-office 
and  calls  the  desired  party.  With  this  method  the  "A"  operator  usually 
supervises  the  connection.  This  plan  of  using  trunks  from  "A  "  to  "B" 
operators  is  of  course  necessary  in  any  system  which  has  more  than  one 
manual  office. 

If  party  lines  are  connected  to  the  sub-office  the  trunk  operator  may 
ring  on  them  selectively  or  by  code,  following  whatever  plan  and  using 
whatever  apparatus  has  been  adopted  for  ringing  on  the  regular  party 
lines  connected  directly  to  the  main  office.  Talking  current  may  be 
supplied  to  the  called  telephone  from  the  main-office  battery  through  the 
trunk  operator's  cord-circuit  relays,  but  it  is  preferable  to  supply  it  from 
the  sub-office  battery  through  the  relays  of  the  connector  switch. 

Interconnections  Between  Sub -office  Lines. — Where  the  trunks 
incoming  to  the  main  office  from  the  sub-offices  are  scattered  among  the 
"A  "  operators,  an  interconnection  is  handled  in  the  same  manner  as  that 
described  for  an  outgoing  call  from  the  main  to  the  sub-office.  Where 
the  incoming  trunks  terminate  in  line  jacks  before  the  operator  at  the 
special  position  equipped  for  handling  outgoing  calls  to  the  sub-office, 
this  operator  responds  to  incoming  trunk  calls  by  plugging  in  to  the  proper 
line  jack,  then  picks  up  the  other  plug  of  the  cord  used,  inserts  it  in  the 
jack  of  an  outgoing  trunk  to  a  connector  switch  in  the  proper  group  at 
the  sub-office  and  from  there  on  handles  the  call  just  as  she  would  any 
outgoing  trunk  connection. 

PLAN  NO.  2 

The  sub-office  equipment  for  use  with  this  plan  may  be  the  same  as 
that  used  with  Plan  No.  1.  The  difference  between  the  two  arrangements 
lies  in  the  equipment  in  the  manual  central  office. 

Incoming  Calls  to  the  Main  Office. — For  this  plan  the  trunks  incom- 
ing to  main  from  the  sub-office  terminate  in  special  cords  and  plugs 
before  the  special  "B,"  or  trunk,  operator.     An  incoming  call  lights  an 


230  AUTOMATIC  TELEPHONY 

associated  lamp.  The  operator  responds  by  pressing  the  answering  key 
associated  with  this  cord  circuit,  takes  the  order,  at  the  same  time  picks 
up  the  plug  and  after  the  busy  test  inserts  it  in  the  multiple  jack  of  the 
desired  party.  The  service  is  speeded  up  appreciably  because  it  is  not 
necessary  to  use  an  answering  plug  and  cord. 

Outgoing  Calls  to  Sub-offices. — It  is  not  necessary  to  have  the  trunk 
operator's  position  equipped  with  line  multiple  jacks.  Therefore  one  of 
the  end  positions  of  the  board,  which  is  not  fully  equipped  with  multiples, 
or  a  separate  desk  may  be  used,  unless  this  operator  handles  the  incoming 
trunk  calls  also.  Each  outgoing  trunk  ends  in  a  key  in  the  "B"  opera- 
tor's position,  and  is  also  connected  to  a  trunk  multiple  jack  in  each 
section  of  the  multiple  switchboard  so  that  each  "A"  operator  will  have 
access  to  it. 

A  call  is  handled  as  follows:  When  the  "A"  operator  receives  the 
order  of  the  calling  main-office  party,  she  presses  a  key  in  her  order  wire 
to  the  trunk  operator's  set  and  repeats  the  number  desired,  for  example, 
"132."  The  trunk  operator  says  "132  on  7"  (No.  7  being  an  idle 
outgoing  trunk  to  the  No.  1  line  switch  group  at  the  sub-office),  and  at  the 
same  time  presses  the  ke>  of  trunk  No.  7,  thus  switching  in  her  calling 
device,  and  pulls  "3-2"  on  the  calling  device  dial.  She  then  presses 
No.  7  ringing  key  to  signal  the  called  party,  unless  the  connector  switch 
is  arranged  to  ring  his  bell  automatically.  Meanwhile  the  "A"  operator 
has  plugged  into  jack  of  trunk  No.  7,  thus  completing  the  connection. 
As  soon  as  she  does  this  a  guard  lamp,  associated  with  trunk  No.  7  in  the 
trunk  operator's  position,  lights  and  remains  lit  until  the  "A"  operator 
pulls  down  the  connection. 

The  "A"  operator's  cord  circuit  lamps  give  her  the  usual  supervisory 
and  clearing  out  signals.  The  "B"  operator  pays  no  attention  to  the 
connection  after  setting  it  up,  or  after  ringing  the  called  party  once  if  the 
ringing  is  done  from  the  main  office.  If  the  called  party  does  not  answer 
promptly,  where  ringing  from  the  main  office  is  used,  the  "A"  operator 
may  ring  him  again  by  pressing  the  proper  ringing  key  in  the  usual  way. 
When  she  receives  the  clearing  out  signal  she  pulls  down  the  connection 
and  when  she  removes  the  plug  from  the  trunk  multiple  jack  the  guard 
lamp  in  "  B"  position  is  extinguished  and  at  the  same  time  the  connector 
switch  used  at  the  sub-office  automatically  releases.  If  the  called  party 
should  be  busy  when  the  trunk  operator  attempts  to  call  him,  the  trunk 
operator  does  not  change  her  method  of  handling  the  connection  and  the 
busy  signal  is  given  instantly  and  automatically  by  the  connector  switch 
to  the  calling  subscriber.  The  circuits  may  be  arranged  for  feeding 
talking  current  to  the  called  party's  telephone  from  the  main-office 
battery  through  the  "A  "  operator's  cord-circuit  relays,  but  it  is  preferable 
from  a  transmission  standpoint  to  have  the  current  supplied  through  the 
relays  of  the  connector  switch  from  the  sub-office  battery. 


AUTOMATIC  SUB-OFFICES  231 

Interconnections  Between  Sub-office  Lines. — For  use  in  such  con- 
nections the  "B"  operator  handling  the  trunks  to  the  sub-office  should 
have  within  her  reach  jacks  associated  with  outgoing  trunks  so  that 
she  can  insert  the  plug  of  the  cord  through  which  she  receives  the  call  into 
an  idle  out  trunk  jack,  and  then  set  up  the  connection  in  the  usual  way. 
The  supervisory  signals  of  the  incoming  trunk-cord  circuit  are  watched  by 
her  just  as  on  an  incoming  trunk  connection  to  a  main-office  line. 

Sub- office  Battery.— A  small  storage  battery  of  suitable  voltage  is 
generally  used  to  furnish  current  for  operating  the  sub-office  switches. 
This  battery  may  be  charged  by  means  of  a  simple  automatic  arrangement 
over  idle  trunks  from  the  main  office,  so  that  it  is  not  necessary  for  an 
attendant  to  go  to  the  sub-office  and  switch  the  charging  current  on  or  off. 
If  desired,  the  battery  may  be  charged  by  a  small  mercury  arc  rectifier, 
or  by  other  means,  installed  at  the  sub-office  and  switched  in  and  out 
of  circuit  by  an  attendant. 

Supervision. — A  sub-office  apparatus  is  usually  equipped  with  tell- 
tale signals  so  that  it  can  be  supervised  quite  easily  from  the  main  office. 
It  is  also  supplied  with  testing  switches,  such  as  are  generally  installed  in 
sub-offices  connected  to  automatic  central  offices  and  by  means  of  which 
the  wire  chief  at  his  central-office  desk  can  make  any  tests  he  desires 
on  any  sub-office  line  without  the  assistance  of  any  one,  either  at  main  or 
the  sub-office.  It  is  not  necessary,  and  it  is  not  customary,  to  keep  an 
attendant  on  duty  in  one  of  these  sub-offices;  in  fact,  the  apparatus 
is  often  left  for  several  days  without  any  attention.  It  should  receive 
a  short,  thorough  inspection  at  regular  intervals.  The  most  complex 
parts  of  the  mechanism  are  the  connector  switches  terminating  the  trunks 
outgoing  from  central  to  the  sub-office,  and  if  one  of  these  should  be 
out  of  order  the  trunk  operator  will  simply  refrain  from  using  it  until  it  is 
repaired. 

Semi-automatic    Sub-office    Using  Lineswitches  and  Connectors  of  the  Automatic 

Electric  Company's  Manufacture  in  Connection  with  Main  Office  Switchboard 

Equipment  of  the  Dean  Electric  Company's  Manufacture 

The  circuits  of  this  system,  which  is  a  representation  of  Plan  No.  1,  are 
shown  diagrammatically  in  Fig.  197.  The  trunk  relay  used  for  closing 
the  circuit  for  energizing  the  bridge  cut-off  relay  winding  of  the  switch 
and  controlling  its  release,  also  for  supplying  talking  current  to  the  calling 
party,  is  somewhat  different  from  the  repeaters  used  in  full  automatic 
practice. 

Incoming  Trunk  Calls. — When  the  lineswitch  extends  the  calling  party's 
line  through  to  the  trunk  relays,  current  flows  from  earth  through  one 
winding  of  the  double  wound  relay  through  the  subscriber's  loop  and  the 
winding  of  the  250-ohm  relay  to  negative  battery.  This  negative  line 
relay  closes  circuit  from  earth  to  the  release  trunk  and  through  the  wind- 


232  AUTOMATIC  TELEPHONY 

ing  of  the  bridge  cut-off  relay  to  negative  battery.  It  also  breaks  its 
circuit  through  the  3100-ohm  supervisory  relay,  which  is  common  to  all 
the  trunks  of  the  group,  and  which  serves  to  open  the  circuit  through  the 
starting  relay  of  the  lineswitch  unit  master  switch,  when  all  of  its  out- 
going trunks  are  busy.  The  object  of  this  is  to  keep  the  master  switch 
from  seaching  for  an  idle  trunk  when  none  is  to  be  found.  The  negative 
line  relay  closes  circuit  also  from  negative  battery  through  the  500-ohm 
coil  to  the  inter-office  section  of  the  trunk.  The  positive  line  relay  coil 
closes  circuit  from  earth  through  the  slow-acting  500-ohm  relay  to  nega- 
tive battery.  This  relay  it  will  be  noted  controls  contact  between  the 
release  trunk  and  the  earth  connection.  The  purpose  of  this  relay  is  to 
prevent  the  lineswitch  from  releasing  when  ringing  current  is  sent  over 
the  line  to  complete  a  reverting  party  line  call. 

At  the  central  office  the  trunk  terminates  in  the  customary  jack  and 
line  relay  equipment  used  for  regular  subscriber  lines.  The  line  relay  is 
operated  by  current  flowing  through  the  positive  line  relay  coil,  of  the 
district  station  trunk  relays,  and  the  500-ohm  winding  of  the  negative 
trunk  relay.  The  operator,  before  whose  position  this  line  terminates, 
responds  to  the  call  as  if  it  were  from  a  regular  manual  subscriber  line. 
She  receives  the  customary  supervisory  signals.  The  lineswitch  does 
not  release  when  the  calling  party  restores  his  receiver  to  the  switch-hook, 
unless  the  operator  has  withdrawn  the  answering  plug  used  from  its 
jack. 

Outgoing  Calls. — When  a  manual  subscriber  wishes  a  connection  to 
an  automatic  sub-office  subscriber,  he  removes  his  receiver  from  its 
switch-hook  and  gives  his  order  in  the  customary  manner  to  the  "A" 
operator  who  responds.  She  repeats  the  order  over  an  order  wire  to  the  "B" 
operator  handling  the  outgoing  trunks  to  the  automatic  sub-office.  This 
procedure  is  the  same  whether  the  "A"  operator  is  in  the  same  office  as 
the  "5"  operator  or  not.  The  "B"  operator  repeats  the  number  and 
assigns  an  idle  trunk  in  the  usual  manner.  At  the  same  time  she  picks 
up  the  trunk  plug  and  inserts  it  in  the  jack  of  an  idle  outgoing  trunk  to 
the  proper  group  of  the  automatic  sub-office,  throws  the  calling-device 
key  and  calls  the  last  two  digits  of  the  sub-office  subscriber's  number. 

The  connector  switch  has  the  customary  relays  and  magnets  except 
that  the  line  relay  has  a  single  winding  only,  one  terminal  of  which  is 
connected  to  earth  and  the  other  to  the  positive  side  of  the  incoming 
trunk.  The  negative  side  of  trunk  is  normally  open  at  side-switch  wiper 
No.  2,  and  this  connector  switch  is  operated  through  earth  over  the  posi- 
tive side  of  the  trunk. 

The  operation  of  a  connector  switch  has  been  explained.  If  the  called 
line  is  busy,  circuit  is  closed  from  guarding  earth  potential  on  the  private 
bank  contact  through  the  private  wiper,  side-switch  wiper  No.  1,  and  one 
coil  of  the  busy  relay  to  negative  battery.     This  relay  closes  a  circuit  from 


AUTOMATIC  SUB-OFFICES  233 

earth  through  the  springs  of  the  1300-ohm  slow  relay,  its  own  locking 
winding  and  the  private  magnet,  which  prevents  the  private  magnet  from 
releasing  its  armature  and  allowing  the  side  switch  to  move  to  third  posi- 
tion. This  relay  also  opens  the  circuit  through  the  rotary  magnet  and 
closes  the  circuit  from  the  "busy"  bus  bar  to  the  negative  side  of  the  line. 
The  busy  signal  is  transmitted  directly  through  the  " B"  and  "A  "opera- 
tors' cord  circuits  to  the  calling  subscriber. 

If  the  called  party  should  not  be  busy  when  the  connector-shaft  wipers 
stop  on  his  bank  contacts,  the  side  switch  moves  to  third  position,  circuits 
are  closed  in  the  usual  manner  for  operating  the  bridge  cut-off  relay  of  the 
called  party's  lineswitch,  and  the  connection  is  extended  through  the 
called  party's  normals  to  his  line  and  telephone. 

The  called  party  is  not  signalled  automatically  by  the  connector 
switch,  but  is  signalled  by  the  "B"  operator  using  her  ringing  keys. 
Since  four-party  line  service  is  given  through  this  sub-office,  and  the 
" B"  operator's  cord  circuits  were  already  equipped  with  four-party  line- 
ringing  keys  when  the  sub-office  was  installed,  it  was  thought  advisable 
to  simplify  the  sub-office  equipment  by  having  the  " B"  operator  do  the 
selective  ringing.  To  make  this  possible  it  should  be  observed  that 
when  the  side  switch  of  the  connector  moves  to  third  position,  the  line  is 
entirely  clear  through  the  connector  with  the  exception  of  the  250-ohm 
line  relay  connected  from  the  positive  side  of  the  line  to  earth,  and  the 
250-ohm  back-bridge  relay  connected  from  the  positive  side  of  the  called 
party's  loop  to  negative  battery,  through  side  switch  No.  4.  When  the 
called  party  responds,  this  back-bridge  relay  is  energized  through  the 
called  party's  loop,  and  the  tip-relay  winding  of  the  " B"  operator's  cord 
circuit.  This  tip  relay  operates  the  supervisory  lamp  cut-off  relay  of  the 
cord  circuit,  while  the  back-bridge  relay  of  the  connector  switch  closes 
circuit  from  the  negative  side  of  the  line  through  a  500-ohm  coil  to  earth. 
The  purpose  of  this  coil  is  to  increase  the  supply  of  talking  current  to  the 
called  subscriber,  and  to  balance  the  line  during  conversation.  When 
the  subscriber  hangs  up  his  receiver,  the  " A"  operator  receives  the  cus- 
tomary disconnect  signals  and  withdraws  the  plug  from  the  trunk  to  "B" 
operator,  who  then  receives  the  disconnect  signal  and  withdraws  the 
plug  from  the  outgoing  trunk  to  the  automatic  switch.  This  breaks  the 
circuit  through  the  positive  line  relay  of  the  connector  switch  and  as  its 
armature  falls  back,  circuit  is  closed  through  the  release  magnet,  resulting 
in  the  release  of  the  switch. 

Interconnections  Between  Sub-office  Subscribers. — The  method  of 
interconnecting  one  sub-office  subscriber  with  another,  by  using  a  trunk 
incoming  from  the  sub-office  for  receiving  the  calling  subscriber's  order 
and  another  trunk  outgoing  to  the  sub-office  for  calling  the  desired  party, 
is  obvious  from  the  preceding  circuit  drawings  and  explanations.  It 
should  be  stated,  however,  that  if  a  subscriber  on  a  party  line  wishes  to 


234  AUTOMATIC  TELEPHONY 

talk  to  another  party  on  his  own  line,  that  a  special  method  of  handling 
the  connection  is  required.  It  is  necessary  for  the  calling  party  to  give 
the  trunk  operator  his  own  number  and  the  number  of  the  party  he  wishes. 
She  then  removes  the  answering  plug  used,  inserts  the  calling  plug  of  an 
idle  cord  in  the  jack  of  the  trunk  on  which  the  party  is  waiting  and  tells 
him  to  hang  up  his  receiver  for  a  moment  while  she  rings  the  desired 
party's  bell.  His  lineswitch  does  not  release,  however,  for  reasons  already 
explained.  The  operator  then  presses  the  ringing  key  coresponding  to 
the  current  frequency  required  to  ring  the  bell  of  the  desired  party,  and 
thus  signals  him  through  the  trunk  used  by  the  calling  party.  After 
having  hung  up  his  receiver  for  a  moment,  as  instructed,  the  calling 
party  removes  it  and  awaits  the  response  of  the  called  party.  This  is 
similar  to  a  method  of  handling  reverting  calls,  which  is  very  common  in 
manual  practice. 

Modification  of  Plan  No.  2. — A  modification  of  Plan  No.  2  is  an  instal- 
lation employing  the  circuits  shown  in  Fig.  198.  The  lineswitch  equip- 
ment is  of  the  Automatic  Electric  Company's  type  and  the  central  office 
equipment  of  the  Stromberg-Carlson  Manufacturing  Company's  make. 
The  lines  witches  and  the  master  switches  used  are  so  similar  to  those 
already  described  that  the  discussion  of  their  circuits  is  not  necessary. 
The  relay  used  on  each  trunk  incoming  to  the  main  office  from  the  sub- 
office  has  coils  through  which  talking  current  is  supplied  to  a  calling 
subscriber  from  the  sub-office  battery.  When  a  lineswitch  extends  a 
connection  to  a  trunk,  current  immediately  flows  through  both  windings 
of  the  double-wound  line  relay  of  the  trunk-relay  set  and  through  the 
subscriber  loop.  This  relay  closes  circuit  from  ground  to  the  release 
trunk,  energizing  the  bridge  cut-off  relay  winding  B.C.O.  of  the  line- 
switch.  The  trunk-line  relay  also  closes  circuit  from  earth  through  the 
1300-ohm  slow-acting  relay,  which  also  controls  a  connection  between 
the  release  trunk  and  earth.  The  purpose  of  this  slow-acting  relay  is  to 
prevent  the  lineswitch  from  releasing  in  case  a  calling  subscriber  moves 
his  switch-hook  up  and  down  repeatedly,  in  order  to  signal  the  operator. 
The  trunk  line  relay  closes  a  circuit  also  between  the  two  sides  of  the 
inter-office  portion  of  the  trunk,  through  the  bridge  coil,  whereupon  the 
main  office-line  relay  operates  and  signals  the  operator,  who  responds, 
takes  the  subscriber's  order  and  completes  the  connection  in  the  manner 
common  to  regular  manual  practice. 

Outgoing  Calls. — When  the  subscriber's  operator  receives  an  order 
for  a  sub-office  number,  she  presses  an  order-wire  key  and  repeats  the 
order  to  the  "B"  operator  in  charge  of  the  outgoing  trunks  to  the 
sub-office.  The  "B"  operator  repeats  the  number  and  assigns  an  idle 
trunk  to  the  "A"  operator,  who  immediately  inserts  the  calling  plug  of 
the  cord  used  to  take  the  order  into  the  corresponding  trunk  jack.  When 
she  does  this,  the  trunk  jack  springs  close  circuit  from  earth  through  the 


AUTOMATIC  SUB-OFFICES  235 

210-ohm  trunk  supervisory  relay,  the  jack  springs,  the  tip  conductor  of 
the  trunk  and  the  500-ohm  relay  G  of  the  connector  to  battery.  The  re- 
sult is  that  the  trunk  supervisory  relay  closes  a  circuit  through  the  guard 
lamp  which  remains  lit  so  long  as  this  particular  trunk  is  occupied,  thus 
preventing  the  "B"  operator  from  reassigning  it.  '  At  the  same  time  that 
the  "B"  operator  assigns  the  trunk  she  throws  the  corresponding  calling- 
device  key,  which  connects  the  calling  device  between  the  negative  side 
of  the  outgoing  trunk  line  and  earth,  and  calls  the  desired  party,  making 
two  turns  of  the  calling-device  dial.  The  circuits  of  the  connector  switch 
are  »so  similar  to  those  which  have  already  been  explained  that  they  will 
not  be  discussed  in  detail.  It  should  be  noted,  however,  that  when  the 
connection  has  been  completed  the  called  party  is  rung  automatically 
by  the  connector  switch,  using  ringing  generator  current  supplied  over  an 
extra  cable  pair  from  the  main  office.  These  circuits  and  those  of  the 
automatic  interrupting  device  for  making  and  breaking  the  circuit 
through  the  ringing  relay  B,  which  is  connected  to  negative  battery  when 
sideswitch  No.  3  moves  to  third  position,  are  shown  in  detail  in  the 
diagram. 

Party  Lines. — It  should  be  noted  also  that  two-party  lines  are  used  in 
this  system,  but  harmonic  ringers  are  not  employed.  The  ringer  of  one 
telephone  on  each  line  is  connected  between  one  side  of  the  line  and  earth, 
and  the  ringer  of  the  other  telephone  between  the  other  side  of  the  line 
and  earth.  To  make  this  practicable,  one  terminal  of  the  ringing  ma- 
chine at  main  office  must  be  permanently  connected  to  earth  and  two 
groups  of  connector  switches  must  be  employed  at  the  sub-office,  one 
group  being  arranged  so  that  its  ringing  relays  will  project  ringing  current 
on  to  the  positive  side  of  a  called  line,  and  the  other  group  to  the  negative 
side  of  the  line.  The  only  circuit  change  that  is  required,  to  accomplish 
this  is  to  reverse  the  line  wiper  connections  on  one  group  of  connector 
switches.  The  necessary  ground  connection  is  secured  at  the  main  office, 
through  the  other  side  of  the  cable  pair,  one  side  of  which  is  connected 
to  the  ringing  generator.  The  return  circuit  of  the  ringing  generator  is 
carried  back  to  central  to  prevent  inductive  disturbances  in  the  cable. 

Supervision. — After  the  " B"  operator  has  set  up  the  connection  she 
pays  no  further  attention  to  it.  If  the  called  line  is  engaged  the  busy 
signal  is  furnished  automatically  by  the  connector  switch.  When  the 
parties  hang  up  the  "A"  operator  receives  the  necessary  disconnect, 
supervisory  signals,  pulls  down  the  cord  circuit  used  and  thereby  causes 
the  release  of  the  connector  switch. 


CHAPTER  X 
THE  SYSTEM  OF  THE  WESTERN  ELECTRIC  COMPANY 

The  Western  Electric  Company  began  work  on  an  automatic  system 
in  1899.  It  was  developed  to  a  commercial  point  in  1910,  and  for  test  a 
semi-automatic  system  of  450  lines  was  installed  in  the  general  offices  of 
the  Western  Electric  Company  in  New  York  City.  This  was  maintained 
and  operated  by  the  New  York  Telephone  Company  as  one  of  its  offices 
in  the  metropolitan  area. 

Early  in  their  development  two  switch  structures  appeared,  the  rotary 
and  the  panel.  The  former  was  characterized  by  having  the  bank  of 
contacts  arranged  on  the  surface  of  half  a  cylinder,  ten  levels  and  20  sets 
of  contacts  per  level.  There  was  a  vertical  shaft  at  the  center  which 
carried  ten  sets  of  wipers  or  brushes.  All  were  connected  in  parallel 
but  only  the  set  which  was  released  by  a  tripping  device  would  engage 
the  bank  when  the  shaft  was  rotated.  The  panel-type  switch  had  all 
the  bank  contacts  arranged  in  a  vertical  row,  and  the  wiper  was  driven 
up  over  the  entire  100  sets. 

About  1911  the  work  was  separated.  The  panel  switch  was  to  be 
developed  in  America,  the  rotary  type  in  Europe.  For  several  years  the 
factory  at  Antwerp,  Belgium,  manufactured  and  installed  a  few  small 
exchanges  in  French  and  English  cities.  The  work  was  stopped  by  the 
German  advance  in  the  war  during  August,  1914.  Since  the  war  ended, 
the  work  has  been  resumed. 

American  development  concentrated  on  the  panel  switch  and  the 
system  to  work  it.  Special  attention  was  given  to  its  use  in  very  large 
cities  like  New  York,  Chicago,  etc.  Much  of  the  trying  out  of  the  appa- 
ratus was  done  in  the  offices  of  the  Newark,  New  Jersey,  exchange,  and 
in  the  semi-automatic  form.  By  this  means  the  operation  from  the 
subscriber's  standpoint  was  not  changed,  and  the  switches  could  be 
tried,  modified,  and  developed  as  much  as  was  desired. 

An  automatic  system  in  a  very  large  city  requires  large  call  numbers, 
often  as  high  as  seven  digits  on  the  decimal  basis.  In  some  cases  there 
may  be  one  more  digit  for  party  line  stations.  In  the  effort  to  reduce  the 
mental  labor  of  remembering  the  call  number  long  enough  to  dial  it,  they 
devised  the  plan  of  spelling  out  the  first  two  or  three  letters  of  the  office 
name,  which  would  be  then  printed  in  bold-faced  type  as  in  Fig.  199. 
The  calling  device  has  three  letters  in  each  number  space  in  addition  to 
the  usual  Arabic  digit.     (Fig.  200).     All  the  letters  of  the  alphabet  are 

236 


Key  Springs  Hartied  j 
X  fo  ma  He  before     ' 


50w>  Req'.CD.    I   I  Reserve    \_ 
i      Comm.toPos..      I- 
fal  switchboard  of  the  Dean  Electi 


THE  SYSTEM  OF  THE  WESTERN  ELECTRIC  COMPANY 


237 


used  except  "Q"  and  " Z. "     The  last  hole  bears  the  digit  "0"  and  the 
word  "OPERATOR." 

The  plan  proposed  is  that  the  subscriber  shall  dial  the  bold-faced 
letters  of  the  office  name,  followed  by  the  four  digits  of  the  number  of  the 
line  in  the  office.  In  smaller  cities  it  may  be  necessary  to  use  only  two 
letters  in  the  office  number.  The  subscriber  will  be  cautioned  not  to 
use  the  tenth  hole  marked  "0"  while  dialing  the  office  name,  but  to  use 
the  sixth  hole  in  which  are  the  letters  "  MNO. "  Also  not  to  mistake  the 
letter  "I"  in  the  fourth  hole  for  the  digit  "  1 "  in  the  first  hole. 


Argon  Dress  Co,  24  E  12 STU  yrsnt  2011 

Argonaut  Supply  Corp,   50   Union  sq.  .STU  yrsnt  7476 
Argonne  Steamship  Co,   17  Battery  pl...RECtor  2493 

Argos  Ad- Art  Co,  1138  Bway FAR  ragut  5986 

Argosy  The    <A  Pub).   280  Bway W0R  th  8800, 


Fig.  199. — Directory  testing  with 
office  names. 


Fig. 


200. — Western  Electric 
calling  device. 


Fig.  201. — Western  Electric 
automatic  telephone. 


Capital  letters  must  be  used  in  the  directory,  to  prevent  confusion 
between  the  figure  "l"  and  the  letter  "l" 

The  complete  desk  telephone  is  shown  in  Fig.  201.  The  post  which 
carries  the  transmitter  is  set  back  of  the  center  of  the  base,  so  as  to  give 
room  for  the  calling  device  to  lie  down  almost  horizontal  (15  degrees 
slope). 

The  general  arrangement  of  apparatus  is  shown  in  Fig.  202. 

All  the  subscriber  lines  terminate  in  the  central  office  on  the  banks  of 
finder  switches  (Line  Finder  Frame)  and  on  the  banks  of  the  connector 
(Final  Frame).  The  line  finder  has  many  vertical  rods,  each  of  which 
carries  several  wipers  or  brushes.  The  latter  are  to  be  lifted  by  suitable 
mechanism  and  made  to  touch  the  contacts  of  a  calling  line,  connecting  it 


238 


A  UTOMA  TIC  TELEPIION  Y 


to  the  first  piece  of  numerical  apparatus,  the  district  selector  (District 
Frame). 

The  distribution  of  traffic  to  the  several  offices  is  made  by  the  district 
selector,  or  by  it  and  the  office  selector  (Office  Frame).  Both  are  me- 
chanically alike. 

The  district  frame  has  five  large  banks,  each  of  which  has  100  sets  of 
contacts  vertically  and  60  sets  horizontally  (30  sets  on  each  side  of  the 
bank).  There  are  60  vertical  rods  (30  on  each  side),  each  of  which  has 
five  sets  of  wipers  or  brushes.  The  bottom  set  of  brushes  engages  the 
bottom  group  of  100  contacts  in  a  vertical  row  in  the  bottom  bank.  The 
next  brush  engages  similarly  the  next  banks,  and  the  third  brush 
the  third  bank,  etc. 

Each  vertical  rod,  with  its  five  brushes,  represents  one  trunk  from 
the  line  finder.     By  suitable  means  only  one  set  out  of  the  five  is  per- 


Ca6ARfiSldiher    GARFIELD  OFFICE 
3345  (Machine  Switching) 


Incoming 
Frame 


Sender    .,, 
Selector  *Y 
\5endei\ 


PENNSYLVANIA  OFFICE 
( "ochine  Switching)  Ca!kcl5ubscr!ber 

Pennsylvania 
5280 


Incoming 
Frame 


Final  Frame 


NORTH  OFFICE 

(  Manual) 


-Called  Subscriber 
NORth  4259 


Call  Indicator  Position 
Fig.  202. — General  plan  of  trunking. 


mitted  to  touch  the  bank  contacts  at  a  time.  At  the  bottom  of  the 
district  frame  is  an  elevator  for  each  rod,  which  lifts  the  rod  and  its 
brushes  so  that  the  brush  which  has  been  rendered  effective  will  wipe  over 
the  bank  contacts  in  its  own  vertical  row. 

Because  this  system  is  primarily  for  very  large  cities,  there  will  be  a 
large  number  of  offices.  In  general,  there  must  be  a  group  of  trunks  out 
of  the  office  under  consideration  leading  to  each  of  the  other  offices. 
Since  the  district  selector  has  only  five  banks  and  five  sets  of  brushes, 
each  bank  cannot  be  set  aside  for  one  office  alone,  unless  it  be  a  nearby 
office  and  the  trunk  group  is  very  large.  Some,  if  not  all,  of  the  banks 
must  be  subdivided  into  smaller  groups  of  trunks.  Also,  the  office 
selector  must  be  used  to  help  the  district  selector  to  reach  so  many  offices. 

Some  of  the  groups  of  trunks  leaving  the  banks  of  the  district  frame 


THE  SYSTEM  OF  THE  WESTERN  ELECTRIC  COMPANY         239 

run  to  brushes  on  the  office  frame,  one  trunk  to  a  selector  with  its  rod 
and  five  brushes.  The  office  frame  has  five  banks,  and  each  of  them 
must  in  turn  have  its  100  sets  of  contacts  subdivided  into  smaller  trunk 
groups.     In  this  way  it  is  possible  to  trunk  to  100  offices,  more  or  less. 

It  is  evident  that  often  two  digits  must  be  used  to  select  a  group  of 
trunks  in  the  district  frame,  one  digit  to  select  the  set  of  brushes  (one  out 
of  five)  and  another  digit  to  cause  the  brush  to  move  to  a  desired  group 
of  trunks  in  the  bank  of  100  over  which  the  brush  can  act.  The  same  is 
true  of  the  office  selector. 

The  first  two  or  three  digits  of  the  call  number  must  control  the  dis- 
trict selector  and  the  office  selector.  These  are  represented  by  the  first 
letters  of  the  office  name,  but  are  translated  into  decimal  digits  by  the 
calling  device  on  the  telephone.  These  two  or  three  decimal  digits  must 
be  translated  into  two  or  four  digits  of  the  kind  which  will  properly 
control  the  selectors.     This  is  done  by  the  sender  in  the  central  office. 

NAME  OF  DIGIT  FUNCTION  CONTROLLED 

u.      ,       ._,                ,  [District  Brush 

Hundred  Tfyousanals) ^District  Group  of  Trunks 

Ten-Thousands         J  I  g££'c<?  £"«"     ._      , 

^U  trice  (?roup  c?f  Trunks, 

Thousands  .^__^^ >  Incoming  Brush 

~"~~x^- v  Jncomina  Group  of  Trunks 

Hundreds^=^ZZ^~~  .      ,  ' 

>-  Final  Brush 

Tens >  Final  Group  of  Ten  Lines 

Units  — — >■  Final  Line  to  Subscriber 

Fig.  203. — Relations  of  call  number  to  switches  and  functions. 

Part  of  the  work  of  the  sender  is  illustrated  by  Fig.  203,  which  gives 
the  relationship  between  the  decimal  digits  of  the  call  number  and  the 
functions  of  the  switches  which  they  control.  Assume  a  six-digit  call 
number,  composed  of  the  digits  whose  names  are  listed  in  the  left-hand 
column  of  the  illustration.  The  first  two  digits  (hundred  thousands  and 
ten  thousands)  must  be  combined  so  as  to  pick  out  the  proper  brush  on 
the  district-selector  frame  and  cause  it  to  move  to  a  definite  group  of 
trunks.  After  the  idle  trunk  has  been  found,  the  sender  must  (according 
to  these  same  two  digits)  choose  the  right  brush  on  the  office-selector 
frame  and  move  it  to  a  certain  group  of  trunks.  It  may  be  for  some  of 
the  calls  that  the  office  frame  may  be  omitted,  and  it  is  even  possible  for 
a  very  few  to  have  only  the  choosing  of  the  district  brush. 

The  thousands  digit  fixes  the  brush  which  shall  be  taken  on  the  in- 
coming selector,  but  it  wil  be  observed  (Fig.  202)  that  each  brush  can 
reach  two  separate  thousands,  so  that  the  relation  is  not  decimal,  although 
it  is  numerical. 

The  thousands  and  the  hundreds  digits  (Fig.  203)  must  be  combined 
to  move  the  brush  on  the  incoming  frame  to  the  right  group  of  trunks,  for 


240  AUTOMATIC  TELEPHONY 

each  brush  can  reach  each  of  the  two  500-line  frames  in  each  of  two 
thousands  (Fig.  202).  Both  the  hundreds  and  thousands  digits  are 
involved. 

The  hundreds  digit  fixes  which  brush  on  the  final  selector  shall  be 
taken  (Fig.  203).  This  part  is  a  simple  choice  along  decimal  lines  (Fig. 
202),  for  each  brush  can  reach  only  100  lines. 

The  tens  and  the  units  digits  (Fig.  203)  go  through  decimally,  for 
bank  on  the  final  frame  (Fig.  202)  must  be  divided  into  ten  groups  of 
ten  subscriber  lines  each. 

Attached  to  each  trunk  from  finder  to  selector  is  another  switch 
called  the  "sender  selector."  It  is  the  duty  of  this  switch  to  hunt  an 
idle  sender  and  connect  it  to  the  trunk,  so  that  the  sender  can  receive 
the  impulses  from  the  calling  device  on  the  telephone,  translate  them, 
and  repeat  them  to  the  district  selectors  and  to  the  office  selector  if  it  is 
used,  also  to  all  the  other  switches  used. 

When  a  call  arrives  at  the  called  office,  it  comes  to  a  selector  on  an 
"incoming  frame."  Mechanically  this  is  like  the  district  frame,  but  the 
bank  is  subdivided  differently. 

It  will  be  observed  in  the  illustration  that  each  of  the  five  banks 
on  the  incoming  frame  is  divided  into  four  parts.  Each  of  these  parts 
contains  trunks  leading  to  a  final  frame,  which  corresponds  to  the  con- 
nector of  other  systems.  There  they  terminate  in  final  selectors,  one 
trunk  to  one  rod  with  its  five  brushes.  In  this  way  one  incoming  frame 
can  reach  final  frames  leading  into  10,000  subscriber  lines. 

The  selecting  of  a  brush  on  the  incoming  frame,  the  moving  of  this 
brush  to  the  necessary  group,  and  the  selection  of  a  brush  on  the  final 
frame,  are  all  non-decimal  actions.  They  are  cared  for  by  the  sender 
in  the  calling  office.  The  moving  of  the  brush  on  the  final  frame  to 
a  group  of  ten  lines,  and  the  last  selection  of  the  individual  set  of  contacts, 
are  decimal  actions. 

If  there  are  manual  offices  in  the  exchange,  the  trunks  will  terminate  in 
relays  and  plugs  on  the  manual  board.  There  is  also  a  call  indicator,  on 
which  the  four  digits  of  the  call  number  will  be  indicated  by  lamps,  so 
that  the  operator  can  read  off  the  number  and  plug  up  the  connection 
with  certainty. 

The  Handling  of  a  Typical  Call. — Assume  the  call  number  to  be 
PENnsylvania  5280.  The  subscriber  will  first  remove  the  receiver  from 
the  hook  and  listen  for  the  dial  tone.  In  this  interval  the  line  finder  has 
seized  his  line  the  sender  selector  has  found  an  idle  sender,  and  the  sender  is 
ready  to  receive  the  call.  This  is  indicated  by  a  "dial  tone"  sent  to 
the  calling  telephone.  The  subscriber  will  insert  his  finger  in  the  open- 
ing of  the  hole  over  the  letter  "P"  rotate  the  dial  until  the  finger  comes 
into  contact  with  the  finger  stop,  remove  the  finger,  and  let  the  dial  auto- 
matically return  to  normal.  He  will  repeat  this  operation  for  the  letters 
"B"  and  "N"  and  for  the  four  digits  5,  2,  8,  and  0. 


THE  SYSTEM  OF  THE  WESTERN  ELECTRIC  COMPANY         241 

The  calling  subscriber  is  assumed  to  be  in  the  GARfield  office.  While 
the  subscriber  is  dialing  as  above  described,  the  calling  device  on  his 
telephone  is  sending  impulses  to  the  "sender"  which  receives  and  regis- 
ters them.  If  the  subscriber,  for  any  reason,  should  fail  to  dial  the  full 
number,  the  selection  which  is  described  later  would  not  take  place, 
but  the  call  would  be  taken  up  by  a  maintenance  employee  and  the  sub- 
scriber told  of  his  error,  unless  he  had  already  hung  up  and  dialed  again. 

The  "sender"  having  received  the  call  number,  causes  the  proper 
brush  on  the  district  selector  to  be  made  operative,  guides  the  elevating 
apparatus  while  it  elevates  the  brush  rod  until  the  selected  brush  reaches 
the  group  of  trunks  leading  to  the  PENnsylvania  office,  and  causes  it  to 
hunt  for  an  idle  trunk  in  that  group.  This  extends  the  connection  from 
the  sender  to  an  incoming  selector  in  the  distant  office. 

The  "sender"  now  controls  the  incoming  selector  in  the  PENnsylvania 
office  very  much  as  it  did  the  district  selector.  It  must  select  a 
brush  and  make  it  operative,  it  must  cause  that  brush  to  be  lifted  to 
a  certain  group  of  contacts,  and  then  guide  it  in  the  hunting  of  an  idle 
trunk  leading  to  a  final  selector  or  connector. 

Lastly,  the  sender  controls  the  final  selector  or  connector  by  selecting 
a  brush,  lifting  it  to  a  certain  group  of  ten  lines,  and  then  causing  it  to 
move  to  the  particular  line  called,  "5280." 

If  the  call  had  been  for  another  GARfield  subscriber,  the  call  would 
have  been  sent  to  an  incoming  selector  in  the  same  office,  and  thence 
to  a  final  selector  (connector). 

When  the  final  selector  has  completed  the  connection,  the  called 
line  will  be  rung  or  a  busy  tone  will  be  sent  back  to  the  calling  subscriber 
if  the  called  line  is  busy. 

APPARATUS  DETAILS 

Line  Finder. — The  line  finder  (Fig.  204)  is  a  long  vertical  frame  which 
contains  the  bank  terminals  for  many  subscriber  lines.  The  general 
functions  are  as  have  been  described  in  an  earlier  chapter  of  this  book. 

Sender  Selector. — The  sender  selector  is  made  up  out  of  rotary 
switches  like  that  shown  in  Fig.  205.  The  bank  has  22  points;  one  is  to 
lead  current  to  the  wipers,  the  rest  are  for  bank  connections.  In  form  it  is 
very  similar  to  the  rotary  lineswitch  described  in  connection  with  Fig.  48. 
Many  sender  selectors  are  built  up  on  one  large  frame  (Fig.  206). 
At  the  right  are  relays  and  resistances  used  in  connection  with  the  sender 
selectors.  To  the  left  of  the  relays  are  the  banks  of  rotary  switches  with- 
out wipers  and  magnets,  as  they  are  here  in  process  of  assembly.  Still 
farther  to  the  left  are  sequence  switches,  used  with  the  selectors. 

District  and  Office  Selectors. — The  district  selector  and  the  office 
selector  are  so  nearly  alike  that  one  description  will  suffice  for  both 
(Fig.  207). 

16 


242 


A  U  TOM  A  TIC  TELEPHON  Y 


In  the  center  are  to  be  seen  the  five  large  banks,  with  30  rods  running 
vertically  across  them,  each  rod  carrying  five  sets  of  brushes.  At  the 
bottom  of  the  bank  space  are  the  elevators  for  lifting  the  rods,  as  well  as 
for  lowering  them  back  to  normal.  Above  the  banks  are  commutators, 
one  for  each  of  the  rods.  On  the  other  side  of  the  banks  are  30  more  rods, 
making  60  selectors  in  all,  which  have  access  to  the  five  banks. 

At  the  right  of  the  banks  are  the 
sequence  switches,  30  of  them,  one  for 
each  brush  rod.  They  are  operated  by 
friction  clutches  from  a  vertical  shaft  at 
their  left. 

To  the  right  of  the  sequence  switches 
are  the  relays  and  resistances  covered 
by  boxes  to  keep  out  the  dust. 

The  bank  itself  (Fig.  208)  is  built  up 
out  of  alternate  strips  of  metal  and  in- 
sulating material.  Above  the  assembled 
bank  is  a  sample  of  each,  the  insulator 
being  above  the  contact  strip.  The 
latter  is  punched  out  with  60  lugs,  30  on 
each  side.     Three  strips  form  a  set  for 


lit,,,. ..."  .mmam. 

PIJ,li.H8.0»ir<J.';?!ifi 


Fig.  204. — Line  finder  frame. 


Fig.  205. — Rotary  switch  for  sender  selector. 


one  trunk  line  or  subscriber  line  (tip,  ring,  and  sleeve).  One  hundred 
sets  of  strips  (300  strips)  are  built  up  into  one  bank.  The  arrangement  is 
such  that  the  lugs  may  be  aligned  vertically  in  rows,  and  groups  of  rows. 
At  the  extreme  left  may  be  seen  three  vertical  rows  of  lugs  or  contacts. 
They  are  the  row  over  which  one  set  of  brushes  move.  The  next  throe 
vertical  rows  of  contacts  are  for  the  next  set  of  brushes.  In  this  way 
30  sets  of  brushes  are  accommodated  on  each  side  of  the  bank — 60  sets  in 
all.     The  total  number  of  contacts  on  both  sides  is  18,000. 


THE  SYSTEM  OF  THE  WESTERN  ELECTRIC  COMPANY 


243 


The  nature  of  this  structure  is  such  that  great  accuracy  of  alignment 
must  be  obtained  and  maintained.  The  bank  must  be  flat  and  the  sides 
of  the  contact  lugs  ground  to  a  straight  line,  because  the  brushes  make 
contact  with  the  edges  of  the  contacts  instead  of  their  flat  sides.     The 


Fig.  206. — Sender  equipment. 


entire  bank  as  a  unit  must  also  be  carefully  adjusted  in  position  and  main- 
tained there,  because  the  brushes  are  on  long  rods  which  are  controlled 
from  the  bottom. 

The  set  of  brushes  (Fig.  209)  consists  of  four  springs;  37  and  38  are 
the  brushes  for  the  "sleeve"  or  third  wire;  36  and  39  are  the  line  brushes. 


244 


AUTOMATIC  TELEPHONY 


Fig.  207. — Selector  frame. 


Fig.  208.— Multiple  bank. 


THE  SYSTEM  OF  THE  WESTERN  ELECTRIC  COMPANY 


245 


All  are  mounted  on  a  carriage  or  frame  which  is  clamped  to  the  vertical 
brush  rod  at  the  left,  inside  of  which  are  the  wires  which  carry  the  circuits 
to  the  brushes. 

Normally  the  brushes  are  kept  apart  by  the  piece  35  which  has  an 
insulator  on  each  end;  34  is  a  lever  used  to  trip  the  spreader  down  so  as 
to  permit  the  brushes  to  rub  on  the  edges  of  the  contacts;  33  is  a  finger 


JJ  JO' 


J7  /O 


Fig.  209. — Selector  brush  and  bank. 

carried  on  rod  32  so  arranged  that  if  rod  32  is  rotated,  the  finger  33  will 
be  above  the  lever  34.  If  now  the  brush  rod  be  lifted,  the  spreader  will 
be  tripped  from  between  the  brushes,  and  the  latter  brought  against  the 
edges  of  the  contacts. 

Figure  210  shows  photographs  of  the  group  of  brushes.     The  two 
lower  ones  show  the  brushes  normal,  the  upper  pair  show  them  tripped. 


(i^'1':.    <TO 


MLc 


Fig.  210. — Selector  brush,  operated  and  non-operated. 

The  two  at  the  right  are  side  views,  while  the  two  at  the  left  are  views 
from  below.  In  the  upper  right-hand  view,  the  long  lever  which  projects 
downwardly  is  the  resetting  lever  which  forces  the  brushes  apart  and  off 
the  contacts  when  the  brush  rod  reaches  the  bottom  of  its  motion  on 
release. 

The  control  of  the  brush  rod  is  shown  more  clearly  in  Fig.  211.     The 
bottom  bank  is  shown  quite  clearly  in  the  upper  half  of  the  photograph. 


246 


A  UTOMA  TIC  TELEPHON Y 


In  front  of  it  are  a  number  of  brush  rods.  The  left  rod  is  missing,  but 
its  brush  trip  rod  is  there.  The  third  rod  is  shown  partly  elevated, 
revealing  the  perforated  strap  by  means  of  which  the  elevating  rolls  lift 
and  lower  tin'  rod.  The  connection  between  the  flat  strap  and  the  round 
rod  is  somewhat  flexible.  The  long,  narrow  slots  in  the  strap  are  to  hold 
the  rod  in  its  operated  position — there  is  in  the  elevator  a  detent  which 
engages  the  strap  and  whose  point  enters  the  slots. 

The  clutch  mechanism  is  shown  more  clearly  in  Fig.  212.     The  brush 
rod  is  11,  at  whose  lower  end  is  the  flat  strap  13.     The  up-drive  roller  15 


Fig.  211. — Selector  frame,  details  of  control  mechanism. 


clears  the  strap  by  very  little  space,  and  is  running  all  the  time.  The 
magnet  19  pulls  levers  24  and  18  so  as  to  cause  roller  16  to  press  the  strap 
against  the  up-roll.  The  detent  which  holds  the  brush  rod  in  the  operated 
position  is  marked  25  and  is  opposite  the  down-drive  roll,  29. 

This  same  illustration  also  shows  more  easily  the  trip  rod  32  and  the 
magnet  31  which  operates  it. 

The  commutator  (Fig.  213)  is  at  the  upper  end  of  the  brush  rod, 
which  is  seen  supporting  the  brush  holder  of  the  brushes  which  engage 
the  commutator.  The  left  part  shows  the  back  of  the  commutator,  with 
the  strips  of  metal  which  carry  the  current  to  the  brushes  on  the  front 


THE  SYSTEM  OF  THE  WESTERN  ELECTRIC  COMPANY 


247 


side  (shown  at  the  right).  The  front  has  three  conducting  strips,  each 
of  which  is  perforated  by  holes  and  slots  of  varying  dimensions.  These 
take  part  in  placing  the  brushes  where  they  belong  during  their  upward 
motion. 

Several  sequence  switches  are  shown  mounted  in   Fig.   214.     The 
constantly  rotating  drive  shaft  is  at  the  left.     On  it  is  mounted  one  hori- 


Fig.  212. — Clutch  mechanism. 


zontal  disc  for  each  sequence  switch.  On  the  shaft  of  the  switch  is  a 
disc  whose  end  is  near  the  disc  on  the  drive  shaft.  A  stationary  electro- 
magnet is  capable  of  magnetizing  these  two  discs,  drawing  them  into 
contact,  and  thereby  causing  a  clutch  action,  rotating  the  sequence 
switch. 


248 


A  UTOMA  TIC  TELEPIION  Y 


Fig.  213. — Commutator. 


Fig.  214. — Sequence  switches  mounted. 


THE  SYSTEM  OF  THE  WESTERN  ELECTRIC  COMPANY 


249 


A  single  sequence  switch  is  shown  in  Fig.  215.     It  consists  of  a  hori- 
zontal shaft  with  many  insulating  discs  on  it.     Each  disc  is  termed  a 


Fig.  215. — Sequence  switch  and  cam. 

"  cam, "  of  which  a  sample  is  shown  at  the  right.  Four  stationary  springs 
or  brushes  engage  each  cam,  and  the  metal  mounted  on  it.  By  it  many 
complicated  connections  can  be  made  and  broken  during  the  progress  of 


Fig.  216. — General  view  of  panel  type  switch  installation. 

setting  up  a  call.  There  is  a  position  indicator  at  the  right,  showing  that 
there  are  18  positions,  each  of  which  is  centered  by  the  cam  and  roller  A 
at  the  left. 


250 


AUTOMATIC  TELEPHONY 


n.i.u.iiiij 


Fig.  217. 


::                                          '.■ 

^    »«i>n       -ft&Sl 

BgjSSjjIffi 

w/^^^^  -^^ShB^s   iS?  '^^mibI 

^^ 

Fig.  218. 


THE  SYSTEM  OF  THE  WESTERN  ELECTRIC  COMPANY         251 

A  general  view  of  selector  frames  installed  is  shown  in  Fig.  216.  The 
cabling  is  carried  overhead  on  runways.  No  attempt  is  made  to  cover 
any  part  of  the  mechanism  except  the  relays. 

CONNECTIONS  TO  MANUAL 

Automatic  to  Manual.' — Automatic  subscribers  dial  complete  numbers 
for  all  calls,  regardless  of  their  destination.  All  alike  are  recorded  on  a 
sender.  If  the  called  number  is  in  a  manual  office  (as  NORth  office  in 
Fig.  4),  it  will  be  switched  to  a  trunk  which  terminates  in  a  plug  on  the 


Fig.   219. — Manual  position  with  push-button  calling  device. 

"B"  operator's  position.  Associated  with  that  position  is  a  "call 
indicator"  on  which  the  last  four  digits  of  the  call  number  are  displayed 
by  lamps.  (See  Fig.  217.)  The  number  here  illustrated  is  "04259." 
The  appearance  of  a  manual  position  equipped  with  a  call  indicator  is 
shown  in  Fig.  218.  The  dark  rectangle  lying  between  the  edge  of  the 
shelf  and  the  row  of  keys  is  the  indicator. 

Manual  to  Automatic— If  the  traffic  to  automatic  offices  from  the 
manual  office  under  consideration  is  large,  the  latter  will  use  push-button 
calling  devices  which  will  be  operated  by  the  "A"  operators  (Fig.  219), 
marked  "Numerical  keys."  A  call  from  a  subscriber  will  be  answered 
as  usual  with  the  answering  plug  of  a  pair.     The  "A  "  operator  will  press 


252 


A  U  TO  MA  TIC  TELEPHON  Y 


the  office  button  which  corresponds  to  the  order-wire  button  used  on 
calls  to  other  manual  offices.  This  will  cause  a  trunk  indicator  to  display 
the  number  of  the  trunk  to  be  used,  into  which  the  operator  will  insert 
the  calling  plug  of  the  pair  used.  She  will  then  operate  the  keys  of  the 
calling  device  for  the  four  digits  of  the  call  number  in  the  called  office. 
This  will  transmit  the  impulses  for  operating  the  switches  in  the  distant 
office. 


Fig.  220. — Cordless  B  board. 

If  the  traffic  to  automatic  is  small,  or  the  manual  office  under  con- 
sideration is  soon  to  be  changed  to  automatic,  the  "A"  operators  will 
not  do  the  calling,  but  it  will  be  done  by  "B"  operators  located  at  a 
"cordless  B"  board  in  the  automatic  office  (Fig.  220).  The  call  is 
passed  to  the  cordless  "B"  operator  who  sets  up  the  four  digits  on  a  push 
button  calling  device  which  has  four  rows  of  push-buttons,  like  an  add- 
ing machine.1 

1  Note. — This  chapter  is  for  the  greater  part  drawn  directly  from  the  publications 
of  the  American  Telephone  &  Telegraph  Company,  by  whose  courtesy  the  illustrations 
are  reproduced.     Reference  was  also  had  to  certain  United  States  patents. 


CHAPTER  XI 
AUTOMANUAL  SYSTEM 

The  automanual  system  combines  automatic  switching  with  manual 
operation  by  interposing  the  work  of  an  operator  between  the  subscriber 
and  the  automatic  switches.  It  consists  of  a  complete  automatic  switch- 
ing installation  in  the  central  office,  ordinary  manual  common  battery 
subscriber's  telephone  connected  thereto,  and  operators'  apparatus  added 
to  enable  the  operators  to  control  the  action  of  the  switches. 

The  idea  of  employing  an  operator  to  receive  instructions  from  a  tele- 
phone user  and  to  operate  automatic  switches  for  completing  the  connec- 
tion is  very  old  in  the  art  of  telephony.  In  the  early  eighties  Connolly 
proposed  it  and  applied  for  a  patent  on  a  crude  arrangement.  As  early 
as  1894  Strowger  automatic  switches  were  operated  by  operators  in 
setting  up  connections  from  a  manual  to  an  automatic  exchange.  In 
1904  Leroy  W.  Stanton  read  a  paper  before  the  International  Electrical 
Congress  at  St.  Louis,  Mo.,  in  which  he  proposed  semi-automatic  opera- 
tion of  multi-office  exchanges. 

In  1906  Edward  E.  Clement  applied  for  certain  patents  related  to  a 
semi-automatic  system,  which  later  developed  into  the  system  made  by 
the  North  Electric  Company  and  sold  by  the  Telephone  Improvement 
Company,  under  the  trade  name  of  "  Automanual."  It  was  put  upon 
the  market  in  1909  since  when  a  number  of  plants  have  been  installed. 
Among  them  may  be  mentioned  Ashtabula,  Ohio,  Galesburg,  111.,  and 
Greensburg,  Ind. 

The  general  layout  of  the  trunking  scheme  is  shown  in  Fig.  221.  The 
subscribers'  lines  at  the  left  are  multipled  to  the  banks  of  finder  switches, 
here  designated  "primary  selector"  switches.  Each  primary  selector  is 
attached  to  a  trunk  leading  to  a  first  selector.  From  the  banks  of  the 
first  selectors,  trunks  run  to  second  selectors  and  from  the  banks  of  the 
second  selectors,  trunks  are  provided  to  the  connectors  in  the  individual 
hundreds.  Associated  with  each  connector  is  a  ringing  selector  switch  for 
party-line  use.  The  latter  switch  delivers  to  the  connector  the  proper  fre- 
quency of  ringing  current  for  ringing  the  bell  of  the  desired  station  on  the 
line. 

The  gain  in  the  efficiency  of  operators  by  the  use  of  traffic  distributing 
apparatus  is  explained  at  length  in  the  chapter  devoted  to  equipment  of 
that  character.  The  automanual  system  includes  the  traffic  distributor 
idea,  but  replaces  the  traffic  distributor  operator's  act  of  picking  up  a 

253 


254 


AUTOMATIC  TELEPHONY 


trunk  ending  plug  and  inserting  it  into  a  multiple  jack,  with  the  act  of 
set  ting  up  the  called  party's  number  on  a  set  of  keys  similar  to  those  of  an 
adding  machine. 

The  traffic  distributor  operator  has  to  make  the  usual  busy  test  and 
pull  out  the  plug  used  when  conversation  is  completed.  These  things  the 
automanual  operator  does  not  do.  The  traffic  distributor  system  is  less 
expensive  than  the  automanual  to  install,  but  which  is  the  more  econom- 
ical to  operate  and  maintain  in  single-office  systems  remains  to  be 
determined. 

The  automanual  is  adapted  to  multi-office  systems  where  considerable 
inter-office  trunking  is  done,  because  the  operator's  work  is  the  same  on  a 
trunkecl  call  as  on  one  completed  locally.     All  the  switches  used  in  the 


Ringing  Selector 
Snitch 
WS7°    /   s7 


Subscriber        Prima 

o- 


V\  Selector 
^V      Switch 


/"Selector  Sws. 


Subscriber     "Q 
+39 


Prima  ry 


L^/r 


Key  Set 


3OT3 


Connector  StfS.         4  Party  Link 


Key 


Set 


— o°°°o    Sw'itches\,°°°  o 

|       Opera  torsi 
| ' — I  Key  Sets* ' — | 


Fig.  221. — Trunking  system. 


automanual  system,  with  the  exception  of  a  few  auxiliary  switches,  arc  of 
the  hundred-point  two-motion  type.  The  bank  contacts  are  set  on  edge 
(vertically)  and  the  shaft  is  arranged  first  to  rotate  to  a  vertical  row  and 
then  to  rise  to  an  individual  contact.  The  auxiliary  switches  are  arranged 
to  be  rotated  by  a  ratchet  action  in  one  direction,  so  that  their  restoration 
to  normal  consists  in  driving  them  on  until  the  wipers  leave  the  bank. 

The  subscribers'  lines  are  grouped  by  hundreds,  each  hundred  lines 
being  served  by  a  number  of  primary  selector  switches  (usually  ten). 
For  each  group  there  is  one  primary  distributing  switch  and  one  key-set 
distributor.  The  function  of  the  former  is  to  find  an  idle  primary  selector 
and  cause  it  to  seek  the  calling  line.  The  function  of  the  latter  is  to  find 
an  idle  operator's  position,  and  to  cause  the  key-set  switch  to  hunt  the 
trunk  line  attached  to  the  primary  selector  which  has  found  the  calling 


AUTOMANUAL  SYSTEM 


255 


line.  By  the  combined  efforts  of  these  switches,  the  subscriber's  line  is 
connected  through  a  trunk  to  a  first  selector  and  an  idle  operator's 
equipment. 

The  operator  is  provided  with  one  or  more  keyboards  through  which 
impulses  generated  by  the  sending  machine  can  be  delivered  to  the 
switches  in  such  a  manner  as  to  set  up  the  connection.  After  the  connec- 
tion has  been  established  the  operator's  apparatus  is  disconnected  so 
that  it  may  revert  to  common  use. 

The  order  of  digits  in  the  called  number  is  as  follows:  The  first  digit 
operates  the  first  selector,  the  second  digit  operates  the  second  selector, 


To  Te/ep/ioneXoj 


To  Banks  of  Primary  Selector 
To   c-3„   To  c-4.  To  c-2C 


To  Keyset  Switch 
'  To  Secondary  Selector 


Fig.   222. — Subscriber's  line  and  distributor  circuits. 

the  third  digit  operates  the.  ringing  selector  and  the  fourth  and  fifth  digits 
operate  the  connector. 

Details  of  Circuits.— The  subscribers'  lines  (see  Fig.  222)  have  line  and 
cut-off  relays.  All  of  the  line  relays,  belonging  to  lines  which  are  in  the 
same  vertical  row  on  the  banks  of  the  primary  selectors,  draw  their  cur- 
rent through  a  common  line  relay,  R-3.  This  is  for  the  purpose  of 
enabling  the  primary  selector  to  stop  at  the  row  containing  the  calling  line. 

The  primary  distributor  switch  S  is  of  the  flat  rotary  type,  having  a 
starting  relay  #-4,  a  stopping  relay  R-5  and  a  magnet  M.     The  key-set 


256  AUTOMATIC  TELEPHONY 

distributor,  S-2,  is  similar  except  that  it  has  three  wipers  and  possesses  a 
starting  relay  R-ll,  a  stopping  relay  R-10,  a  magnet  M-2  and  an  auxiliary 
relay  R-9. 

The  primary  selector  (Fig.  224)  and  the  first  selector  are  linked  to- 
gether. Primary  selector  apparatus  is  shown  at  the  left  and  first 
selector  apparatus  at  the  right.  The  line  circuit  is  broken  by  two  con- 
densers and  talking  current  supplied  to  the  calling  subscriber  through 
two  relays  R-14,  and  relay  R-15.  There  are  four  wipers:  the  line  wipers, 
W-3  and  W-4;  the  row  wiper,  W-26,  and  the  individual  wiper,  W-25. 
The  line  wipers  and  the  individual  wiper  are  set  on  edge.  The  row 
wiper,  W-26,  is  set  flatwise  so  as  to  engage  the  flat  contacts,  of  which 
there  are  ten,  one  for  each  vertical  row.  When  the  shaft  moves  upward, 
the  row  wiper  is  lifted  away  from  the  row  contacts. 

The  primary  selector  is  started  over  a  control  wire,  38.  The  two 
relays,  R-20  and  #-21,  are  connected  through  wires  135  and  130  to  the 
secondary  selector  and  thence  to  the  sending  machine.  Impulses  from 
the  latter  operate  the  rotary  relay  R-21,  and  through  it  the  rotary  magnet 
M-8.  In  like  manner  impulses  through  the  vertical  relay  R-20  actuate 
the  vertical  magnet  M-7. 

The  selection  of  a  non-busy  trunk  is  accomplished  by  sending  ten 
impulses  over  line  135,  the  foot-step  or  off-normal  switch,  144-145,  being 
open  and  relay  R-41  closed  to  the  private  wiper  W-147  at  the  time. 
When  a  non-busy  trunk  is  found  relay  R-41  falls  back  and  cuts  off  further 
impulses. 

The  key-set  switch  (see  Fig.  225)  has  line  wipers,  W-SO  and  WS1, 
over  which  conversation  between  operator  and  subscriber  will  take 
place  and  impulses  sent  to  the  connectors  and  selectors.  The  row  wiper 
is  W-82  and  the  individual  test  wiper  is  147-84.  The  wiper  83  is  auxiliary 
and  is  used  for  operating  trunk  cut-off  relay  R-42,  Fig.  224,  as  well  as 
other  relays. 

The  operator's  set  has  a  listening  relay,  33,  which  connects  it  to  the 
circuit.  At  the  right  is  an  auxiliary  switch,  *S-3,  which  has  three  wipers 
driven  by  the  magnet  M-15.  Its  chief  function  is  the  delivery  of  the 
proper  impulses  from  the  sending  machine  and  keyboard  to  lines  131  and 
132,  and  thence  to  the  selecting  switches.  In  the  lower  right-hand 
corner  of  the  diagram  are  shown  a  number  of  relays  which  co-operate  with 
each  other  in  the  control  of  the  impulses.  The  release  magnet  for  the 
key-set  switch,  M-1Q,  belongs  properly  with  the  apparatus  shown  at  the 
left. 

The  connector  switch  (see  Fig.  229)  has  two  line  relays,  R-23  and  R-22. 
The  lines  are  transposed  before  entering  the  switch  circuit.  There  is  a 
rotary  off-normal  spring,  F-6,  and  a  vertical  foot  switch  237  and  238. 
The  rotary  foot  switch,  F-Q,  operates  on  the  first  rotary  step  and  the  verti- 
cal foot  switch,  237-238,  on  the  first  vertical  step.     The  two  control  relays, 


AUTOMANUAL  SYSTEM 


257 


R-24  and  R-27,  co-operate  with  the  line  relays  and  the  foot  switches  in 
delivering  the  impulses,  first  to  the  magnet  M-ll  for  the  frequency 
selector  S-7,  then  to  the  rotary  magnet  M-9,  and  lastly  to  the  vertical 
magnet  M-10.  The  busy-test  relay  is  #-29.  If  the  called  line  is  busy, 
R-29  will  release  the  connector  and  supply  the  busy  tone  to  the  calling 
subscriber.  The  ringing  relay,  R-28,  is  wired  in  the  usual  manner  except 
that  its  sleeve  contact  is  wired  through  a  resistance  r-1  to  negative  battery, 
to  hold  up  the  cut-off  relay  while  ringing. 


To  Primary  Selectors 
and  First  Selectors 


Multiple,  , 


Multiple  toother  Secondary 
Selectors 


No  I  Operator's  Position 
Bonk  of  Keyset  Switch 


Release  JLSr^ 

Wire  +        Busy  CJ2GXD 

jH'I'l1      f  \^   V/-82 


Ttrr 


of  KeyserSn 


Mease.  ♦,,,*»»        *F® 


S-82 
Row  TestConfacts 


Fig.   223. — Relation  between  distributors  and  keyset  switches. 


The  frequency  selector,  S-7 ,  is  a  flat  rotary-type  switch.  The  select- 
ing wiper  is  W-S-70  but  there  are  two  other  wipers,  W-S-  700,  for  the 
purpose  of  restoring  the  switch  to  normal,  by  propelling  it  around  the 
and  W-S-7,  which  has  for  its  object  the  release  of  the  connector. 

The  general  relation  between  the  secondary  distributor  and  the  key- 
set switches  is  shown  in  Fig.  223.  At  the  left  are  shown  the  banks  of  two 
key-set  distributors,  one  of  which  is  assumed  to  be  for  the  first  hundred 
subscribers'  lines  and  the  other  for  the  second  hundred.  For  1000  sub- 
scribers there  would  be  ten  key-set  distributors.  In  the  upper  part  of 
the  figure  are  shown  two  banks,  each  belonging  to  the  key-set  switch  of  an 

17 


258  AUTOMATIC  TELEPHONY 

operator.  These  are  hundred  point  banks  and  are  mulipled  together. 
Each  operator  can,  therefore,  be  connected  to  any  one  of  100  trunks,  the 
same  being  divided  into  ten  groups  of  ten  each.  The  groups  are  by 
vertical  rows. 

When  a  call  is  initiated  in  any  hundred,  the  key-set  distributor  rotates 
to  find  an  idle  operator's  position.  Wire  79  indicates  by  the  presence  or 
absence  of  negative  battery  potential  whether  the  position  is  free  or  busy. 
Position  No.  1  is  shown  as  busy  and  position  No.  2  as  free.  When  the 
primary  distributor  closes  contact  R-10  the  secondary  distributor  will 
rotate  until  wiper  W-S-200  finds  negative  battery  potential  on  wire  79. 
Then  the  distributor  will  stop  and  the  control  relays  R-10  and  R-32  per- 
form their  functions.  Wires  79  are  common  starting  wires,  and  are  mul- 
tipled  to  the  banks  of  all  the  secondary  distributors  which  have  access 
to  these  operators.     The  release  wire  221  is  also  common. 

•  The  row  test  wires  98  are  individual  to  the  hundred  group.  All  those 
proceeding  from  the  key-set  distributor  for  the  first  hundred  will  termi- 
nate on  contact  No.  1  $-82  of  the  row  test  contacts  of  all  the  secondary 
selectors.  Similar  wires  98,  proceeding  from  the  secondary  distributor 
of  the  second  hundred,  are  attached  to  contact  No.  2  on  each  key-set 
switch  bank.  Immediately  above  each  row  test  contact  *S-82  is  the  verti- 
cal row  containing  the  terminals  of  the  trunks  which  serve  the  particular 
hundred  to  which  the  row  test  contact  belongs. 

When  the  key-set  switch  operates,  its  wipers  rotate  until  W-82  strikes 
the  contact  carrying  negative  battery  potential  from  wiper  W-20  and 
contact  R-10.  The  wipers  will  then  be  lifted  until  IF-84  finds  the  indi- 
vidual contact  leading  to  the  trunk  which  at  this  moment  is  busy. 

DETAILED  CIRCUIT  OPERATIONS 

Initiation  of  Call. — When  the  subscriber  takes  the  receiver  from  the 
hook,  the  line  relay  R-l  (Fig.  222)  and  the  row  relay  R-3  both  energize. 
The  line  relay  locks  itself  and  the  row  relay.  The  line  relay  grounds  the 
individual  test  contact  C-25  of  all  the  primary  selectors  in  this  group. 
The  row  relay,  R-3,  grounds  the  row  test  contact,  C-26,  lights  a  super- 
visory lamp  and  energizes  the  starting  relay,  R-A,  of  primary  distributor 
switch. 

Relay  R-A  prepares  the  testing  circuit  of  the  primary  distributor 
switch  by  connecting  relay  R-5  from  ground  to  the  wiper.  It  also  con- 
nects the  interrupter  to  the  magnet  M  so  that  the  wiper  of  the  primary 
distributor  is  thereby  driven  over  the  bank  contacts.  The  wire  38, 
leading  to  an  idle  trunk,  has  negative  battery  potential  on  it,  so  that  when 
the  idle  trunk  is  found,  relay  R-5  will  be  enerigized  in  series  with  a  relay 
connected  with  the  trunk.  The  energizing  of  relay  R-5  cuts  off  the 
current  from  the  magnet  M  and  stops  the  primary  distributor  switch. 

The  same  current  which  stops  the  primary  distributor  switch  energizes 


AUTOMANUAL  SYSTEM 


259 


the  control  relay  R-13  (Fig.  224)  of  the  primary  selector,  both  R-5  and 
.ft-13  being  in  series. 

The  control  relay  on  pulling  up  locks  itself  to  negative  battery,  through 
a  back  contact  on  the  trunk  cut-off  relay  R-42.  The  control  relay  places 
negative  battery  on  wire  45,  leading  to  the  key-set  switch  bank  (Fig.  225) 
contact  *S-84,  which  is  the  individual  contact  in  a  certain  vertical  row,  and 
is  for  the  purpose  of  stopping  the  wiper  of  the  secondary  selector  at  the 
proper  trunk.  The  control  relay  also  connects  ground  from  the  inter- 
rupter I  through  a  back  contact  of  relay  R-12  to  the  rotary  relay  R-1S 
which  also  receives  its  negative  battery  connection  through  the  control 
relay. 


Primary   Selector 
5 


,  «_Jr-|3, 

i     n777T)i 

C^Tcont, 
— 1       Re. 


S5 
First  Selector 


trot 

Relay 


Ui,i,u 


F* 


Vert  Mag.    Rot. 
Mag. 


W  10,     £!P 


c-197 


§^5\        *     ^< 
^  S-S  «:£  ° 

•*>.<>  ">:;<» 

IS*     P 

107 

Fig.  224. — Primary  unci  first  selector  circuits. 

The  pulsations  furnished  by  the  interrupter  I  cause  the  rotary  relay  to 
vibrate  so  that  the  rotary  magnet  N-5  rotates  the  wipers.  During  this 
time  the  row  test  relay  R-12  lies  in  a  circuit  between  the  row  test  wiper 
W-2Q  and  negative  battery  which  it  receives  from  the  control  relay 
through  a  contact  on  relay  7M5.  When  the  subscriber  initiated  the  call 
his  group  relay,  R-3  (Fig.  226),  placed  a  ground  on  the  row  test  contact, 
C-26  (Fig.  222),  of  the  primary  selector,  hence  when  the  primary  selector 
wipers  have  rotated  to  the  row  in  which  the  calling  subscriber's  line 
terminates,  the  row  test  relay  R-12  will  find  ground  and  be  energized. 


260 


A  UTOMA  TIC  TELEPIION Y 


On  pulling  up,  relay  R-12  will  lock  itself  to  ground,  connecting  individ- 
ual test  relay  .ft-15  (Fig.  224)  to  the  wiper  W-2b  and  shift  the  pulsations 
from  the  rotary  relay  to  the  vertical  relay  72-19.  The  vertical  magnet  at 
once  steps  the  shaft  upward,  while  R-15  tests  each  individual  contact 
C-25.  On  arriving  at  the  contact  belonging  to  the  calling  line,  the  indi- 
vidual test  relay  R-15  finds  ground  which  has  been  placed  there  by  the 
line  relay  R-l.  On  pulling  up,  the  test  relay  R-15  cuts  off  the  negative 
battery  supplied  from  the  vertical  relay  R-19  and  the  row  test  relay 
R-12,  so  that  both  of  them  become  de-energized,  stopping  the  wipers  of 
the  primary  selector  on  the  calling  line. 


Fig.   225. — Operator's  keyset  switch  circuit. 

The  line  wipers,  W-3  and  W-4,  are  now  in  contact  with  the  bank  con- 
tacts, C-3  and  C-4.  The  cut-off  relay  of  the  calling  line  is  pulled  up, 
owing  to  the  switching  of  the  individual  test  relay  .R-15  from  the  wiper 
W-25  to  the  line  6,  so  that  current  will  flow  from  negative  battery  through 
the  winding  of  #-15  back  contact  of  R-12,  the  line  6,  wiper  W-4,  contact 
C-4,  winding  of  cut-off  relay  R-2  to  ground.  The  pulling  up  of  the  cut-off 
relay  clears  the  line  of  the  line  relays  R-l  and  R-S,  unlocks  them  from 
ground  and  connects  the  tip  and  sleeve  of  the  subscriber's  line  through 
to  the  primary  selector. 

The  extension  of  the  subscriber's  telephone  line  to  the  primary  selector 


A  U  TOM  A  NUAL  S  YSTEM 


261 


results  in  the  flow  of  current  through  the  subscriber's  telephone  from  the 
two  relays  #-14  and  #-15,  with  the  cut-off  relay  #-12,  tapped  off  to 
ground  from  the  negative  or  sleeve  side  of  the  line.  The  tip  relay  #-14 
on  pulling  up  cuts  off  the  tip  line  7  from  the  trunk  cut-off  relay  #-42. 

Coincident  with  the  stopping  of  the  primary  distributor  switch  (Fig. 
222)  the  key-set  distributor  is  started  on  its  hunt  for  an  idle  operator's 
position.  The  pulling  up  of  relay  R-5  energizes  starting  relay  #-11. 
The  rotary  magnet  M-2  drives  the  wipers  over  the  bank.  Relay  #-11 
also  connects  up  the  testing  circuit  from  ground  through  relay  #-10  to 
the  wiper  W-S-200.  Wire  79  leads  to  the  control  relay  #-32  (Fig.  225) 
of  the  key-set  switch.  If  the  position  is  not  busy  and  is  ready  for  the 
reception  of  a  call,  the  control  relay  will  be  connected  to  negative  battery 
through  back  contact  of  rotary  foot  switch  FS,  terminal  344,  back  con- 
tact of  relay  #-49  of  special  first  selector  (Fig.  232),  terminal  343  key-set 


Subscribers'  I  mes 


Fig.   226. — Identifying  the  calling  line 


switch  (Fig.  225)  contact  on  operator's  receiver  jack.  If  an  operator 
leaves  her  position  she  will  withdraw  the  receiver  plug  cutting  off  the 
negative  potential  from  the  control  relay  and  wire  79  and  making  her 
position  busy. 

When  the  wipers  of  the  secondary  distributor  (Fig.  222)  arrive  at  a 
line  operator's  position,  the  stopping  relay  #-10  will  be  pulled  up  in  series 
with  the  control  relay  #-32  (Fig.  225).  In  the  secondary  distributor 
switch  this  energizes  relay  #-9  and  cuts  off  the  rotary  magnet  M-2  so 
that  the  switch  stops.  Relay  #-9  puts  negative  battery  potential  on 
wiper  W-S-20,  wire  98  leading  to  bank  contact  S-82  which  is  the  row  test 
contact.  Relay  #-9  also  grounds  W-S-200,  which  short-circuits  relay 
#-10,  allowing  the  latter  to  fall  back.  This,  however,  does  not  affect 
relay  #-9,  since  the  latter  is  locked  to  negative  battery  through  its  own 
contact. 

The  pulling  up  of  the  control  relay  #-32  of  the  key-set  switch  (Fig. 
225)  locks  its  own  winding  to  negative  battery  through  a  back  contact  of 
relay  #-30,  prepares  the  row  test  circuit  from  ground  through  a  front 


262  AUTOMATIC  TELEPHONY 

contact  on  relay  R-32,  winding  of  relay  R-31  to  wiper  W-82,  and  closes 
the  interrupter  circuit  from  ground  through  the  interrupter  /,  front 
contact  of  relay  R-32,  winding  of  relay  R-39,  back  contact  of  relay 
R-31,  back  contact  of  relay  #-30  to  negative  battery. 

Pulsations  are  now  delivered  through  the  rotary  relay  R-39  and  thence 
relayed  to  the  rotary  magnet  M-13  so  that  the  wipers  of  the  key-set  switch 
rotate,  hunting  for  the  row  in  which  lie  the  contacts  of  the  trunk  seized. 
When  wiper  TF-82  arrives  at  the  live  contact  £-82,  relay  R-31  pulls  up, 
cutting  off  ground  from  the  rotary  relay  and  giving  it  to  the  vertical  relay 
instead.  At  the  same  time  relay  R-31  locks  itself  to  negative  battery 
through  a  back  contact  of  relay  R-  and  opens  a  wire  leading  from  wiper 
Tf-83. 

The  vertical  magnet  M  no  the  shaft  while  the  individual  test 

relay,  being  connected  between  yer  JF-84  and  ground,  tests  the  indivi- 
dual trunk  contacts.  The  com  J  relay  of  the  primary  selector  seized 
(see  Fig.  224,  relay  R-13)  placed  negative  battery  potential  on  wire  45 
which  terminates  on  the  bank  contact  $-84  of  the  secondary  selector. 
Hence,  on  arriving  at  this  contact,  individual  test  relay  R-30  of  the  second- 
ary selector  will  pull  up,  cutting  negative  battery  current  from  the  vertical 
relay  R-40  and  unlocking  relay  R-31.  The  wipers  are  thereby  stopped 
and  the  circuit  from  wiper  W-S3  closed. 

At  this  moment  two  wires  are  extended  from  the  first  selector  trunk  in 
Fig.  224  to  the  operator's  position.  Wire  130  carries  the  circuit  from  the 
tip  wiper  W-9  of  the  first  selector,  through  a  back  contact  of  relay  R-41, 
through  wire  130,  bank  S-80  of  key-set  switch  (Fig.  225)  and  wiper  W-SO 
to  wire  131.  The  other  is  from  wiper  W-10  of  the  first  selector  (Fig.  224) ; 
through  a  back  contact  of  relay  R-41;  wire  135;  bank  contact  v  °1  of  key- 
set switch  (Fig.  225);  wiper  W-81  to  wire  132. 

The  falling  back  of  relay  R-31  of  the  key-set  switch  close,  a  circuit 
which  simultaneously  energizes  the  trunk  cut-off  relay  R-42  of  the  first 
selector  and  signal  relay  R-34  at  the  operator's  position.  This  circuit 
extends  as  follows:  from  negative  battery,  through  front  contact  of 
sleeve  relay  R-15,  primary  selector  (Fig.  224),  winding  of  trunk  cut-off 
relay  R-42,  wire  107,  bank  contact  £-83  of  key-set  switch  (Fig.  225), 
wiper  TF-83,  back  contact  of  relay  jR-31,  back  contact  of  relay  R-35, 
winding  of  signal  relay  R-34,  back  contact  of  listening  relay  R-33,  back 
contact  of  starting  relay  R-36,  another  back  contact  of  relay  R-35,  back 
contact  of  relay  R-32  (this  having  been  de-energized),  to  ground. 

The  trunk  cut-off  relay  R-42,  on  pulling  up,  cuts  off  the  wires  7  and  8, 
so  that  the  impulses  to  be  sent  will  not  annoy  the  subscriber.  It  also 
unlocks  control  relay  R-13,  as  well  as  stopping  relay  R-5  of  the  primary 
distributor  (Fig.  222).  When  this  latter  relay  falls  back,  current  is 
cut  off  from  relay  R-ll  of  the  secondary  distributor.  This  allows  relay 
R-9  to  fall  back  and  take  the  negative  battery  potential  from  wire  98 


AUTOMANUAL  SYSTEM 


263 


and  row  test  contact  $-82  of  the  secondary  selector.  The  primary  and 
secondary  distributor  switches  are  therefore  returned  to  common  use  so 
that  any  other  subscriber  in  the  same  hundred  may  initiate  a  call. 

The  unlocking  and  falling  back  of  the  contl  relay  72-13  of  the  priminary 
selector  (Fig.  224)  takes  the  negative  battery  potential  from  wire  45  so 
that  any  other  key-set  switch  will  not  stop  on  this  trunk.  The  pulling 
up  of  the  signal  relay  72-34  (Fig.  225)  at  the  operator's  position,  lights  a 
guard  lamp  L-2  (Fig.  227)  which  attracts  the  attention  of  the  operator. 
It  also  rings  a  night  alarm  bell  for  night  service,  if  desired,  and  operates  a 
call  register,  E. 


d"       d"      d'        d2       d5        d 


Fig.  227. — Sending  machine  and  keyset  circuit. 

The  operator  now  answers  the  call  by  pressing  the  listening  key  L-K 
(Fig.  227).  Current  then  flows  from  ground  (Fig.  181)  contact  of  key 
L-K  and  wire  125  to  (Fig.  225)  winding  of  listening  relay  72-33,  back 
contact  of  relay  72-31,  back  contact  of  relay  72-30  to  negative  battery. 
The  listening  relay  immediately  pulls  up  and  locks  itself  to  ground.  It 
breaks  the  circuit  of  the  trunk  cut-off  relay  72-42  (Fig.  224),  which  falling 
back  connects  the  talking  circuit  to  the  operator's  position.  The  signal 
relay  is  also  de-energized  so  that  the  lamp  L-2  is  extinguished.  The  listen- 
ing relay  also  connects  the  operators'  set  to  the  line. 

The  operator  now  speaks  to  the  subscriber  and  obtains  the  desired 
number,  which  she  sets  up  on  the  rows  of  keys  shown  in  Fig.  227.     The 


264  AUTOMATIC  TELEPHONY 

row  of  keys  marked  "K-l"  indicates  the  thousands  digit,  K-2  the  hun- 
dreds digit,  K-3  the  tens  digit,  K-4  the  units  digit  and  K-5  the  station 
desired  upon  a  party  line.  Lastly  she  presses  the  starting  key  S-K 
which  energizes  the  starting  relay  #-36  (Fig.  225).  The  circuit  over 
which  this  is  done  is  as  follows:  ground,  starting  key  S-K  (Fig.  227), 
wire  140,  winding  of  relay  72-36  (Fig.  225),  back  contact  of  relay  #-35, 
back  contact  of  relay  #-31,  wiper  W-83,  contact  £-83,  wire  107  to  (Fig. 
224,)  winding  of  relay  #-42,  front  contact  of  relay  #-15  to  negative  battery. 
This  pulls  up  the  trunk  cut-off  relay,  disconnecting  the  calling  sub- 
scriber and  connecting  negative  battery  through  winding  of  relay  #-41 
to  ground  at  foot  switch  144-145.  Relay  #-41,  therefore,  pulls  up  and 
connects  the  operating  wires  135  and  130  to  the  vertical  and  rotary  relays 
#-20  and  #-21.  Relay  #-41  will  constitute  part  of  the  trunk  seeking 
circuit. 

The  pulling  up  of  relay  #-36  of  the  key-set  switch  (Fig.  225)  unlocks 
the  listening  relay  #-33,  and  disconnects  the  operator's  telephone.  The 
same  main  spring  now  locks  the  starting  relay  in  an  energized  condition, 
using  the  same  ground  as  formerly  held  the  listening  relay  #-33.  The 
starting  relay  puts  negative  battery  on  wire  121  which  relights  the  guard 
lamp  7,-2.  It  will  remain  lighted  until  the  sending  machine  has  completed 
its  work. 

#-36  also  closes  a  circuit  from  the  magnet  M-15  which  operates  the 
auxiliary  switch  S-3,  through  a  back  contact  of  relay  #-35,  front  contact 
of  relay  #-36,  wire  151  to  the  sending  machine  (Fig.  227),  to  the  pair  of 
springs  marked  d-11.  Since  the  shaft  upon  which  all  these  cams  are 
mounted  is  in  constant  rotation,  presently  cam  D-11  will  close  the  springs 
d-11,  sending  one  impulse  to  the  magnet  M-15,  rotating  the  wipers  of  the 
auxiliary  switch  S-3  to  the  first  contact. 

The  auxiliary  switch  prepares  the  circuit  for  the  "thousands "  impulses 
as  follows:  from  the  common  wire  of  the  key  K-l  (Fig.  227),  through  wire 
173,  wiper  W-S-3  (Fig.  225),  back  contact  of  relay  #-1000,  wire  131,  the 
wiper  1^-80,  contact  £-80,  wire  130,  front  contact  of  relay  #-41,  winding 
of  relay  #-21  to  negative  battery. 

As  soon  as  cam  D-11  has  broken  the  contact,  the  number  cams  from 
D-l  to  D-10  inclusive  pass  under  their  contact  springs  d-1  to  d-10  in- 
clusive. These  cams  are  of  graded  length,  D-l  maintaining  contact 
during  only  one  impulse  of  the  interrupter  7.  Cam  D-2  holds  its  contact 
closed  through  two  impulses,  interrupter  cam  7)-3  through  three  impulses, 
and  so  forth.  The  result  is  that  the  key  in  row  K-l  which  is  closed  will 
cause  to  be  delivered  to  wire  173  as  many  impulses  generated  by  the 
interrupter  I  as  correspond  to  the  thousands  digit.  This  will  cause  the 
rotary  relay  #-21  of  the  first  selector  (Fig.  224)  to  attract  its  armature 
the  same  number  of  times,  and  thereby  to  operate  the  rotary  magnet  and 
rotate  the  shaft  to  the  desired  vertical  row.     At  the  first  rotary  step,  of 


AUTOMANUAL  SYSTEM 


265 


the  first  selector  the  rotary  foot  switch  F-5  is  closed,  lighting  the  off 
normal  lamp  L-5  and  preparing  the  circuit  for  the  release  magnets  MS 
and  M-4,  which,  however,  can  not  pull  up  on  account  of  the  circuit  being 
broken  by  the  tip  relay  #-14. 

The  selection  of  an  idle  trunk  is  as  follows: 

After  the  number  cams  have  caused  the  rotation  of  the  first  selector 
wiper  shaft,  contact  d-11  is  again  closed,  stepping  the  auxiliary  switch 
S-3  to  its  second  position  which  connects  wire  189  to  wire  132.  Wire 
189,  it  will  be  observed,  leads  to  the  sending  machine  (Fig.  227)  where  it 
terminates  in  spring  contacts  d-12.  This  is  actuated  by  a  long  cam  which 
keeps  the  circuit  closed  during  ten  impulses  from  the  interrupter  I. 
Ten  pulsations  are,  therefore,  delivered  by  the  impulse  machine  over 
wire  189,  wire  132,  wiper  W-Sl,  contact  S-Sl,  wire  135,  front  contact  of 
relay  #-41,  vertical  relay  #-20  to  negative  battery.     This  actuates  the 


09 


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Release 
Magnet 


Fig.   228. — Second  selector  circuit. 


vertical  magnet,  lifting  the  wipers  on  the  first  selector  in  the  desired 
vertical  row.  The  first  step  of  the  shaft  upward  opens  the  foot  switch 
144-145  so  as  to  give  relay  #-41  the  opportunity  of  testing  the  contacts 
C-197  for  an  idle  trunk.  As  long  as  busy  trunks  are  encountered  current 
will  flow  through  the  testing  relay.  When  private  wiper  PT-147  of  the 
first  selector  finds  no  ground,  relay  #-41  will  release,  cutting  off  further 
pulsations  from  the  vertical  magnet,  stopping  the  selector  upon  a  free 
trunk. 

The  circuits  of  the  second  selector  are  now  prepared.  By  falling  back, 
relay  #-41  grounds  the  private  wiper  JF-147  and  pulls  up  the  control  relay 
R-52  of  the  second  selector  over  the  following  circuit:  Ground,  release 
magnets  M-3  and  M-4  in  parallel,  back  contact  of  relay  #-41,  private 
wiper  W-147,  contact  C-197,  low  resistance  release  magnet  M-22  of  second 
selector  (Fig.  228),  foot  switch  386-387,  winding  of  relay  #-52  to  negative 
battery.  The  latter  relay  on  pulling  up  prepares  the  line  circuits  371 
and  372  for  delivering  impulses  to  the  rotary  and  vertical  relays  #-50  and 


2(H) 


A  U  TOM.  1  77  < •  TELEPHON  Y 


72-51,  with  the  exception  that  the  latter  relay  is  cut  off  by  the  relay 
foot  switch  392-393. 

The  switches  are  now  ready  for  the  "hundreds"  impulses.  Springs 
d-11  of  the  sending  machine  close  again,  causing  the  auxiliary  switch  S-Z 
to  move  to  its  third  position.  This  connects  up  wire  173-a  (Fig.  225)  to 
wire  131,  which  now  delivers  the  hundreds  impulses  to  rotary  relay  72-50 
of  the  second  selector  which  repeats  them  to  the  rotary  magnet  il/-20, 
rotating  the  second  selector  to  the  desired  row  of  trunk  contacts. 

The  second  selector  finds  an  idle  trunk  as  the  first  selector  did.  Cam 
7)-ll  causes  the  moving  of  auxiliary  switch  S-S  to  contact  4  and  follows 
with  ten  trunk  testing  impulses  from  spring  <7-12.  This  lifts  the  wipers  of 
the  second  selector.     The  testing  circuit  includes  relay  72-52,  which  must 


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Fig.  229. — Connector  circuit. 


depend  for  ground  upon  the  private  contacts  C-200  over  which  the  private 
wiper  is  moving.  When  an  idle  trunk  is  found,  relay  72-52  will  fall  back, 
cutting  off  further  vertical  impulses  and  connecting  the  line  wires  through 
to  the  seized  connector.  It  grounds  the  private  wire  leading  to  the  con- 
nector, which  protects  the  trunk  from  being  seized  by  another  second 
selector. 

The  frequency  selector  is  next  operated.  The  auxiliary  switch,  S-S 
(Fig.  225),  is  moved  one  step  as  before,  so  that  its  wipers  rest  upon  contact 
5.  This  connects  the  frequency  impulses,  wire  197  (Fig.  227),  to  the  wire 
131  of  the  secondary  selector  (Fig.  225).  By  this  means  the  number  of 
impulses  corresponding  to  the  station  number  of  the  called  telephone 
will  be  delivered  over  the  wire  131,  wire  130,  wiper  TF-9  of  first  selector, 


AUTOMANUAL  SYSTEM  267 

wire  371  to  second  selector,  wiper  W-130,  contact  C-13  to  connector 
(Fig.  229),  back  contact  relay  #-26,  through  the  winding  of  relay  #-23, 
to  negative  battery.  The  impulses  delivered  to  this  relay  are  repeated 
by  its  grounded  main  spring  to  the  magnet  ilf-11  which  actuates  the  fre- 
quency selector  S-7. 

The  first  impulse  of  relay  #-23  closes  the  control  circuit  of  the  con- 
nectors by  causing  relays  #-24  and  #-27  to  lock  themselves  to  the  release 
trunk,  which  has  just  been  grounded  by  relay  #-52  of  the  second  selector. 
Relay  #-24  locks  itself  through  a  back  contact  on  relay  #-22,  relay  #-27 
energizes  through  front  contact  of  relay  #-23,  and  back  contact  of  relay 
#-25,  to  release  trunk  and  ground.  It  locks  through  the  back  contact  of 
relay  #-29. 

The  connector  switch  is  the  next  to  be  operated.  The  sending 
machine  moves  the  auxiliary  switch  S-S  (Fig.  225)  to  contact  6,  in  which 
position  wire  224,  which  furnishes  the  tens  impulses,  is  connected  to  wire 
132  for  actuating  the  connector  switch.  The  impulses  are  sent  from  the 
impulse  machine,  through  wiper  W-S-30,  wiper  TF-10,  wiper  TF-140,  back 
contact  of  relay  #-26,  winding  of  relay  #-22,  front  contact  of  relay  #-27 
(locked),  back  contact  of  relay  #-29,  to  negative  battery.  The.  vibra- 
tions of  relay  #-22  deliver  impulses  to  the  rotary  magnet  M-9,  which 
rotate  the  shaft  to  the  desired  vertical  row.  The  first  movement  of  relay 
#-22  unlocks  relay  #-24. 

The  auxiliary  switch  S-3  is  now  moved  to  position  7  for  the  purpose  of 
stepping  the  connector  wipers  up  to  the  called  line.  The  units  impulses 
are  furnished  from  the  sending  machine  (Fig.  227),  wire  241  through 
wiper  W-S-3,  wiper  TF-80,  wiper  W-9,  wiper  TF-130,  back  contact  of  relay 
#-26,  through  relay  #-23,  to  negative  battery.  The  first  movement  of 
relay  #-23  again  locks  relay  #-24.  Since  relay  #-27  is  still  locked  ener- 
gized and  the  rotary  foot  switch  F-Q  has  been  moved  from  its  normal  posi- 
tion, theimpulses  now  generated  by  relay  #-23  will  be  sent  to  the  vertical 
magnet  through  front  contact  of  relay  #-27.  The  vertical  magnet  will 
lift  the  shaft  to  the  desired  line  in  response  to  the  definite  impulses  from 
the  sending  machine.  During  the  journey  of  the  wipers  upward  they  are 
cut  off  from  the  tip  and  sleeve  relays  #-25  and  #-26,  respectively,  by 
relay  #-24.  The  called  line  is  tested  by  relay  #-29,  which  is  brought  into 
use  by  relay  #-22.  After  the  number  cams  have  finished  giving  impulses 
and  before  cam  D-ll  closes  its  springs,  cam  D-13  closes  springs  cZ-13 
sending  a  single  test  impulse  to  pull  up  relay  #-22.  It  connects  the  test 
relay  #-29  to  the  wiper  W-1S  as  follows:  beginning  at  wiper  W-18,  back 
contact  of  relay  #-28,  front  contact  of  relay  #-22,  winding  of  relay  #-29, 
release  trunk  back  to  second  selector,  contact  C-200,  wiper  IF-200,  back 
contact  of  relay  #-52,  to  ground.  The  sleeve  side  of  the  subscriber's 
line  is  grounded  through  the  cut-off  relay  #-2  (Fig.  222).  If  the  line  is 
not  busy  no  current  will  be  flowing  through  the  cut-off  relay,  and  there- 


268  AUTOMATIC  TELEPHONY 

fore  the  sleeve  contact  at  the  banks  of  the  connectors  will  have  ground 
potential.  If,  however,  the  line  is  in  use,  the  sleeve  contact  will  be  raised 
above  ground  potential. 

If  the  called  line  is  busy  relay  #-29  will  be  energized  and  lock  itself 
directly  to  negative  battery.  It  will  close  the  circuit  of  the  release 
magnet  M-12,  and  cause  the  connector  to  release  quickly.  The  same 
relay,  #-29,  will  also  unlock  relay  R-27,  so  that  it  will  fall  back,  discon- 
necting the  vertical  magnet  ilf-10  and  reconnecting  the  circuit  of  the 
magnet  M -11  which  belongs  to  the  frequency  selector  S-7.  Since  the 
rotary  foot  switch  has  been  reset  to  normal,  the  circuit  of  the  magnet  for 
S-7  is  now  completed  as  follows:  negative  battery,  foot  switch  F-Q  in  the 
normal  position,  winding  of  magnet  M-ll,  back  contact  of  relay  R-27, 
wiper  W-S-700,  of  the  frequency  selector  through  its  contacts,  to  the 
interrupter  I  to  ground.  The  interrupter  will  send  pulsations  to  magnet 
M-ll  and  cause  the  rotation  of  the  frequency  selector  wipers  until  they 
again  reach  their  normal  position.  At  the  same  time  relay  R-29  connects 
the  busy  tone  to  the  sleeve  line  so  that  calling  subscriber  is  notified  that 
the  line  is  busy. 

When  the  calling  subscriber  hangs  up,  his  tip  relay  #-14  falls  back, 
connecting  battery  to  the  release  magnets  of  the  primary  and  first  selec- 
tors in  parallel  with  the  release  magnet,  of  the  second  selector.  This 
releases  the  primary,  first,  and  second  selectors. 

If  the  called  line  in  this  particular  system  is  not  busy,  relay  R-29 
will  receive  no  current,  and  after  the  transitory  impulse  to  R-22  is  past, 
the  latter  will  fall  back  and  connect  up  the  sleeve  relay  R-2Q  to  wiper 
W-18  which  is  now  resting  upon  the  sleeve  contact  C-4  of  the  called  line. 
Current  will,  therefore,  flow  through  the  sleeve  relay  and  cut-off  relay  of 
the  called  line,  energizing  both.  The  cut-off  relay  will  clear  the  called 
line  except  for  its  own  winding.  The  sleeve  relay  of  the  connector  cuts 
off  both  of  the  relays  R-22  and  #-23  and  connects  the  talking  circuit 
through  the  wipers  W-17  and  W-18. 

The  ringing  relay  now  receives  impulses  from  the  interrupter  7-2 
through  the  back  contact  of  relay  R-24,  front  contact  of  relay  R-27  (still 
locked),  back  contact  of  relay  R -24,  back  contact  of  relay  R-22  to  nega- 
tive battery.  Each  time  relay  R-28  pulls  up  it  gives  the  conditions  of 
Fig.  230. 

When  the  called  station  answers  (during  the  de-energization  of  the 
ringing  relay  R-28)  current  will  flow  to  the  called  telephone  by  way  of  the 
tip  relay  R-25  returning  through  the  sleeve  relay  #-26.  The  pulling  up 
of  the  tip  relay  cuts  off  the  ringing  current  by  unlocking  relay  R-27 
which  breaks  the  circuit  of  the  ringing  relay  #-28. 

In  order  to  guard  the  connector  switch  from  being  seized  in  case  the 
calling  subscriber  should  release  before  the  called  subscriber  hangs  up, 
the  tip  relay  #-25  places  a  ground  upon  the  release  trunk  which  terminates 


AUTOMANUAL  SYSTEM 


269 


in  contacts  C-200  on  the  banks  of  the  second  selectors.  In  this  case  the 
calling  subscriber  would  cause  the  release  of  all  the  switches  except  the 
connector,  but  the  ground  held  on  the  release  trunk  by  tip  relay  R-25 
would  protect  the  connector  circuit. 


Fi<;.   230.- — Rinsing  conditions. 

During  conversation  the  existing  circuits  are  as  shown  in  Fig.  231. 
The  calling  subscriber  draws  current  through  two  relays  in  the  primary 
selector.  The  called  subscriber  is  supplied  with  current  from  two  relays 
in  the  connector.  The  cut-off  relay  of  each  line  is  energized  by  current 
from  the  sleeve  line. 


I    Primary     First  ' 

,  .  I     Selector    Selector    I 

Line     w;j 


VJ'3 


Second 
Selector 


I 

wjuo 


Connector 


W*I7 


Line 


,WS-700 


M^,        ^JyFrequenct/ 
^&AS~r      Selector 


3M  12 

3  Release  Mog  for 
Connector 


r       fr 


Fig.  231. — Talking  connection  tetween  stations. 

The  release  magnets  of  the  primary  selector,  first  selector  and  second 
selector,  are  in  one  common  circuit,  broken  only  at  contact  of  the  relay 
/?-14,  which  will  be  closed  whenever  the  calling  subscriber  hangs  up  his 
receiver. 

The  release  circuit  of  the  connector  is  complicated  by  being  linked  in 
with  the  release  of  the  frequency  selector  to  secure  slow  release.     If  the 


270 


AUTOMATIC  TELEPHONY 


called  subscriber  accidentally  opens  the  circuit  of  his  telephone  by  unin- 
tentional movements  of  the  hook  switch,  his  tip  relay  22-25  will  momen- 
tarily close  the  contact  marked  R-2o  in  the  circuit  of  the  rotary  magnet 
M-ll  of  the  frequency  selector.  This  connects  the  impulse  machine  /  to 
the  rotary  magnet  Af-11  and,  if  the  circuit  should  be  closed  long  enough, 
would  drive  the  wipers  of  the  frequency  selector  on  around  the  circle  to 
normal.  Momentary  movements  of  the  hook  switch  will  not  advance 
the  wipers  of  the  frequency  selector  very  far.  When  the  called  subscriber 
hangs  up  his  receiver  for  a  sufficient  time  the  frequency  selector  will 
reach  normal.  On  the  last  contact  the  circuit  of  the  release  magnet  M-12 
of  the  selector  is  closed  by  wiper  W-S-7  (see  Fig.  229  as  well  as  Fig.  231). 
When  the  subscribers  release,  each  controls  only  a  part  of  the  complete 
connection.     The  calling  subscriber  releases  primary,  first,  and  second 


$47       346.     345      344      343       342      341 

Fig.  232. — Selector  for  reverting  calls. 

selectors.  The  .  called  subscriber  releases  the  connector  as  above 
described. 

Reverting  Calls. — When  a  subscriber  desires  another  subscriber  who  is 
on  the  same  party  line,  the  operator  will  tell  him  to  hang  up  his  receiver 
for  a  moment.  This  will  clear  his  line.  The  operator  will  then  call  the 
desired  subscriber  by  using  a  special  first  selector.  When  the  called 
subscriber  answers,  a  guard  lamp  will  indicate  the  fact  to  the  operator, 
who  will  then  release  the  special  first  selector  leaving  the  subscribers  to 
hold  conversation  on  current  supplied  by  the  connector.  If  desired,  it 
can  be  arranged  so  that  the  operator  can  hold  the  connection  and 
supervise  it. 

The  special  first  selector  (Fig.  232)  has  the  usual  rotary  and  vertical 
relays  and  magnets  R-52,  R-53,  Af-16  and  Af-17,  and  a  switching  relay 
R-50.     This  arrangement  is  almost  exactly  the  same  as  found  in  the  first 


A  UTOMA  N  UAL  S  YS  TEM 


271 


and  second  selectors.  Relay  72-50  also  controls  the  circuit  of  the  private 
wiper  IF- 120. 

The  terminals  numbered  341  to  347,  inclusive,  are  connected  directly 
to  the  similarly  numbered  terminals  in  Fig.  225  at  the  operator's  position. 
"346"  and  "347"  are  the  operating  and  talking  wires.  "343"  and 
"344"  constitute  the  loop  to  make  the  position  busy  by  the  removal  of 
negative  battery  potential  from  wire  79  which  leads  to  a  bank  contact  on 
the  key-set  distributor  switch.     "342"  is  the  control  wire. 

To  use  the  special  first  selector,  the  operator  presses  key  K-7.  This 
pulls  up  relays  72-49  and  72-50.  Relay  72-50  on  energizing  connects  the 
operating  wires  346-347  to  the  vertical  and  rotary  relays  and  connects 
the  winding  of  relay  R-50  to  the  private  wiper  to  be  used  as  a  trunk- 
finding  circuit.  Relay  72-49  opens  the  protective  loop  343-344  making 
the  position  busy,  cuts  off  the  circuit  of  the  release  magnet  M-15  and 
connects  negative  battery  to  the  controlling  circuit  at  the  operator's 


/nfer-  Office  Trunk    c-7 


Manual  Subscribers* 
Line 


Aufo.  Manual 
Selectors 


Fig.  233. — Trunk  to  manual  office. 


position.  The  current  flow  in  this  circuit  is  as  follows:  negative  battery, 
front  contact  of  relay  72-49,  wire  345  to  Fig.  225,  wiper  TF-83,  back  contact 
of  relay  72-31,  back  contact  of  relay  72-35,  winding  of  relay  72-36,  wire  140, 
to  the  impulse  machine  (Fig.  227),  where  it  terminates  on  one  of  the 
springs  of  the  starting  key  S-K.  It  will  be  noted  that  this  circuit,  so  far 
as  the  key-set  switch  is  concerned,  is  the  same  as  was  previously  traced. 
The  operator  sets  up  the  number  by  pressing  the  buttons,  and  finally 
the  starting  key  S-K.  This  energizes  the  starting  relay  72-36  over  the 
circuit  just  described  which  locks  and  causes  the  transmission  of  the 
impulses,  exactly  as  was  described  for  the  regular  connection,  the  only 
difference  being  that  the  impulses,  instead  of  going  out  over  the  wipers 
TF-80  and  TF-81,  pass  over  the  wires  346  and  347,  to  the  special  first 
selector. 

Trunking  from  Automanual  to  Manual. — A  trunking  circuit  for  hand- 
ling calls  from  an  automanual  to  a  manual  office  is  shown  in  Fig.  233. 
At  the  left  are  seen  the  banks  of  first  selectors,  it  being  assumed  that  the 


272 


AUTOMATIC  TELEPHONY 


Fig.  234. — 500-line  installation. 


Fig.  235. —  Wipers  and  magnet  of  rotary  switch. 


AUTOMANUAL  SYSTEM 


273 


trunks  between  offices  are  handled  by  the  first  selector,  requiring  only  a 
single  call  figure  to  establish  the  connection.     The  trunk  terminates  as 
usual  at  the  "B"  board  in  the  manual  office,  under  the  charge  of  a  "B" 
operator,  who  is  expected  to  receive 
her  instructions  over  the  trunk  line. 
For  the  latter  purpose  she  is  provided 
with   the    usual    listening    key    K-Q, 
with    which  is   associated   a  manual 
ringing    key.     The    subscriber's   line 
circuit  shown  at  the  right  is  of  the 
simplest  type,  merely  to  indicate  the 
possession  of  line  and  cut-off  relays. 
It  is,    however,   shown  for  a   three- 
conductor  jack  and  plug. 

When  the  operator  receives  a  call 
for  the  manual  office,  she  causes  a 
first  selector  to  seize  a  trunk  line. 
Upon  the  seizure  of  the  line,  current 
flows  from  the  private  wiper  of  the 
selector  through  the  contacts  C-200  to 
the  center  of  a  bridge  coil  whose  two 
windings  are  indicated  by  "r-2"  and 
"r-3."  The  current  thus  divides  the 
passes  over  both  sides  of  the  trunk 
circuit  to  the  cord  apparatus  in  the 
manual  office  and  passing  through 
the  back  contacts  of  relay  R-45,  goes  through  the  two  windings  of 
relay  R-44  in  parallel  to  negative  battery.  Relay  72-44  immediately  pulls 
up  and  by  means  of  its  own  main  springs,  r-442  and  r-443,  connects  its 


Fig.  236. — Rotary  switch. 


Fig.  237.— Relay. 


windings  directly  to  the  line,  independent  of  relay  R-45.     The  movement 
of  main  spring  r-441  causes  the  lighting  of  the  lamp  L-10. 

The  "B"  operator  in  the  manual  office,  on  receiving  the  number, 

18 


274 


A  U  TOM  A  TIC  TELEPHON  Y 


makes  the  busy  test  in  the  usual  way  and  if  the  line  is  free  inserts  the  plug 
in  the  jack.  This  closes  the  sleeve  circuit  through  cut-off  relay  /?-48, 
sleeve  of  jack  and  plug,  and  lamp  L-9  and  winding  of  relay  R-45  in  par- 
allel. The  lamp  lights  as  a  guard  for  ringing  purposes.  Relay  RA5  cuts 
off  the  lamp  L-10.  The  "B"  operator  now  rings  with  the  ordinary 
ringing  key. 

When  the  called  subscriber  answers  he  will  draw  current  through  relay 
RAQ  which,  on  pulling  up,  will  put  out  the  lamp  L-9,  thus  notifying  the 


Fig.  238. — Wipers  and  magnets  of  switch  unit. 

"  B  "  operator  that  conversation  has  begun.     The  two  lamps  L-9  and  L-10 
are  the  individual  supervisory  signals  for  the  two  subscribers. 

When  the  called  subscriber  hangs  up,  relay  RA6  will  fall  back  and 
light  the  lamp  L-9.  When  the  calling  subscriber  hangs  up  it  causes  the 
release  of  the  first  selector.  This  allows  relay  ZM4  to  fall  back,  whereupon 
the  circuit  of  the  lamp  L-10  is  closed  through  main  spring  r-441,  back 
contact,  to  the  front  contact  of  relay  R-45  main  spring  r-452,  through  the 
sleeve  side  of  the  trunk  circuit  to  main  spring  r-442,  through  its  back 
contact,  through  the  front  contact  of  main  spring  r-453.     Upon  seeing 


AUTOMANUAL  SYSTEM 


275 


both  the  lamps  L-9  and  L-10  lighted,  the  " B"  operator  will  pull  down  the 
connection,  which  will  allow  the  cut-off  relay  .R-48  and  the  trunk  relay 
jK-45  to  fall  back. 

If  desired,  the  number  of  the  called  subscriber  in  the  manual  office  can 
be  transmitted  by  impulses  and  set  up  before  the  " B"  operator,  who  then 
needs  only  to  read  off  the  number  and  make  the  connection  accordingly. 

A  small  installation  of  five  hundred  lines  is  shown  in  Fig.  234.  The 
operator's  desk  may  be  seen  in  the  center  foreground.     It  is  equipped  with 


SECTION  x-x 
Fig.   239. — Side  view  of  2-motion  switch. 

one  key  set,  having  three  digits  (rows  of  keys)  and  an  additional  row  of 
five  keys  for  party  lines. 

A  rotary  switch,  such  as  used  for  the  distributor  switches  and  the  fre- 
quency selector,  is  shown  in  Fig.  235  and  236.  The  magnet  has  a  knife- 
edge  pivoted  armature,  retained  by  coiled  springs,  adjusted  by  screws 
in  the  armature.     The  bank  has  20  points. 

The  line  relay  (Fig.  237)  has  a  heel  piece  which  makes  two  bends,  one 
end  carrying  the  core  and  the  other  used  for  mounting.     The  armature  is 
pivoted  to  the  back  end  of  the  magnet,  through  a  hole  in  the  heel  piece 
Its  finger  projects  forward  to  operate  the  springs. 


270 


A  U  TOM  A  TIC  TELE  I 'HON  Y 


In  the  view  of  a  switch  unit  (Fig.  238)  the  wiper  shaft  is  at  the  left. 
The  rotary  ratchet  wheel  is  pinned  to  the  shaft  and  rotates  and  rises  with 
it.  The  rotary  detent  is  in  the  form  of  a  long  plate  which  holds  the  wheel 
no  matter  how  high  the  shaft  rises.  The  vertical  ratchet  rack  is  attached 
to  the  shaft  by  collars.  It  rises  with  the  shaft,  but  does  not  rotate.  The 
rack  for  the  vertical  detent  is  at  right  angles  to  the  rack  for  the  vertical 
magnet.     The  magnet  coils  are  shown  at  the  right.     The  release  magnet, 


Fig.  240. — Plan-view  of  2-niotion  switch. 


at  the  top,  hides  the  rotary  magnet.  Below  them  is  the  vertical  magnet. 
Further  details  of  the  switch  are  shown  in  the  drawings  of  Figs.  239 
and  240.  In  these  the  banks  are  attached.  The  upper  is  the  line  bank, 
through  which  the  telephone  lines  are  connected.  The  lower  bank  carries 
the  auxiliary  circuits,  which  have  been  described.  All  bank  contacts  are 
set  vertically  (on  edge)  with  the  exception  of  the  top  row  of  the  bottom 
bank.  It  is  the  "row  test"  set  which  is  engaged  by  row  test  wiper,  w-26. 
When  the  switch  has  rotated  and  found  a  certain  row,  the  vertical  motion 
of  the  shaft  lifts  the  row  test  wiper,  it>-20,  clear  of  the  bank. 


CHAPTER  XII 
LONG  DISTANCE  AND  SUBURBAN  EQUIPMENT 

Connections  between  long  distance  lines  and  automatic  switchboard 
telephone  systems  are  set  up  by  operators,  following  quite  closely  the 
methods  current  in  good  manual  telephone  practice.  The  services  of 
these  operators  are  required  for  switching  the  long  distance  lines  for 
"through"  or  " toll-to-toll "  connections,  putting  up  the  connections 
between  toll  and  local  lines,  checking  conversation  lengths  and  for  record- 
ing names  of  parties  to  each  conversation,  the  amount  of  the  fee  charged, 
etc.  Where  a  long  distance  board  is  large  enough  to  require  the  services 
of  a  number  of  operators,  the  circuits  are  usually  so  arranged  that  the 
operators  are  divided  into  classes;  that  is,  recording,  line,  pay  station, 
suburban  and  rural  line  operators  are  used  as  in  manual  practice. 

Since  it  is  impossible,  within  the  confines  of  this  volume,  to  describe 
in  detail  the  circuits  and  apparatus  used  for  long  distance  and  rural  line 
service  in  connection  with  automatic  switchboards  of  each  of  the  makes 
treated  within  these  pages,  it  has  been  thought  best  to  limit  the  chapter 
to  an  exposition  of  some  of  the  Automatic  Electric  Company's  typical 
equipments,  circuits  and  practices.  It  is  felt  that  general  methods  and 
fundamental  principles  are  fully  illustrated  by  limiting  the  chapter  to  one 
system. 

Variations  in  the  circuits  and  equipment  are  largely  due  to  variations 
in  the  means  used  for  furnishing  supervision  to  operators  setting  up 
connections  through  the  automatic  switchboards.  The  following  means 
have  been  employed  to  indicate  when  a  called  party  answers: 

1.  Talking  current  started  to  flow  in  the  calling  party's  loop. 

2.  Direction  of  talking-current  flow  reversed  in  the  calling  party's 
loop. 

3.  Strength  of  current  is  changed  in  callings  party's  loop. 

4.  Current  is  caused  to  flow,  stopped  flowing,  reversed  or  varied  in 
strength  over  a  third  wire  used  for  supervisory  purposes  only  between  the 
connector  switch  and  the  toll  board. 

Recording  Methods. — It  is  usually  the  practice  in  the  exchanges 
employing  Automatic  Electric  Company's  apparatus  for  an  automatic 
subscriber  to  secure  a  connection  to  the  long  distance  switchboard  for 
the  purpose  of  recording  his  order,  by  turning  his  dial  from  the  finger 
hole,  which  is  labeled  "Long  Distance"  as  mentioned  in  the  description 

277 


278  AUTOMATIC  TELEPHONY 

of  the  telephones.  These  words  are  usually  printed  in  connection  with 
the  naught  (tenth)  finger  hole.  Sometimes  other  numbers  are  used, 
such  as  100,  110,  etc.,  depending  upon  the  numbering  scheme.  In  any 
case  the  subscriber  secures  an  idle  trunk  to  a  recording  position  of  the 
long  distance  board.  The  trunks  usually  terminate  in  the  recording  posi- 
tion in  relay  and  lamp  signals  and  are  generally  arranged  so  that  the  oper- 
ator may  respond  by  simply  throwing  a  key.  Sometimes  jacks  are  used 
instead  of  keys.  Details  of  a  typical  recording  operator's  circuit  will  be 
discussed  later.  These  circuits  are  arranged  so  that  when  the  operator 
responds  they  do  not  reverse  the  direction  of  current  flow  through  the 
calling  party's  telephone,  so  that  if  he  is  calling  from  a  measured  service 
line,  he  does  not  have  to  pay. 

In  most  modern  systems  it  is  customary  to  use  what  are  called  "Dis- 
criminating tone  tests"  in  connection  with  recording  operator's  circuits. 
The  purpose  of  this  feature  is  automatically  to  supply  to  the  recording 
operator  a  tone  signal  which  will  warn  her  that  the  request  for  a  long 
distance  connection  comes  from  a  line  belonging  to  a  subscriber  whose 
credit  is  so  poor  that  he  is  not  allowed  long  distance  connections.  When 
thus  warned  the  operator  will  refuse  to  set  up  the  connection,  or  will  refer 
him  to  the  proper  company  official.  A  different  tone  is  used  for  warning 
her,  if  the  call  comes  from  a  subscriber's  station,  whose  proprietor  insists 
that  all  orders  for  long  distanct  connections  must  be  approved  by  him 
personally.  Another  tone  may  be  used  to  inform  the  recording  operator 
when  a  call  comes  from  a  pay  station,  so  that  she  may  make  note  of  this 
fact  on  the  ticket,  which  she  passes  to  the  line  operator,  who  sets  up  the 
connection. 

After  having  made  out  this  ticket  the  recording  operator  tells  the 
calling  subscriber  to  hang  up  his  receiver  and  that  ho  will  be  called  when 
the  party  he  desires  has  been  secured.  When  the  line  operator  has 
secured  the  desired  party  she  calls  the  local  subscriber  and  puts  the  two 
into  connection  with  each  other. 

The  methods  followed  in  setting  up  the  long  distance  connection  may 
be  the  same  as  those  used  in  handling  long  distance  lines  in  connection 
with  manual  telephone  systems  or,  if  the  order  is  for  a  subscriber  in 
another  city,  which  is  equipped  with  an  automatic  telephone  system,  it 
generally  promotes  efficiency  to  have  the  line  operator's  position  equipped 
with  a  calling  device  and  to  have  the  long  distance  line  terminate  in  the 
automatic  switchboard  as  well  as  in  the  toll  board  at  the  distant  end, 
so  that  the  line  operator  can  set  up  the  connection  by  manipulating  her 
calling  device  and  without  the  aid  of  the  operator  at  the  other  end.  It 
has  been  found  that  in  this  way  many  more  connections  can  be  handled 
than  by  the  ordinary  double  checking  method.  Of  course  the  economy 
is  not  so  great  in  comparison  with  a  line  operated  by  the  single  checking 
method. 


LONG  DISTANCE  AND  SUBURBAN  EQUIPMENT  279 

Toll  Line  Connecting  Methods. — Three  different  arrangements  are 
used  for  enabling  toll  line  operators  to  complete  connections  to  local 
lines: 

1.  A  calling  device  and  a  trunk  like  a  subscriber's  line. 

2.  A  calling  device,  toll  selectors,  and  toll  connector  (one  or  more  per 
100  lines). 

3.  A  UB"  operator's  switchboard  to  which  all  subscriber's  lines  are 
multipled. 

The  first  of  the  three  methods  is  the  cheapest  to  install  and  furnishes 
much  more  liberal  trunking  facilities  than  method  2;  in  fact,  where  2  is 
used  it  is  necessary  to  resort  on  occasions  to  method  1  to  take  care  of 
overflows. 

Method  2  is  preferable  to  1  because  connections  are  set  up  more 
quickly  and  better  transmission  is  secured.  When  this  method  is  used 
in  a  multi-office  system,  it  improves  transmission  by  eliminating  one  or  " 
more  repeaters.  A  third  advantage  of  this  plan  is  that  it  is  practicable 
to  arrange  the  toll-connector  switch  so  that  the  ringing  is  under  the  con- 
trol of  the  operator.  This  feature  is  quite  helpful  in  setting  up  toll 
connections,  because  it  enables  the  line  operator  (without  ringing)  to  hold 
a  connection  against  a  possible  call  from  some  local  subscriber,  while  the 
operator  finishes  setting  up  the  long  distance  connection. 

In  working  out  method  2  it  has  become  customary  to  use  several 
combined  toll  and  regular  connectors  in  each  hundred  line  board,  arrang- 
ing them  so  that  they  are  the  first  choice  of  the  toll  selectors,  but  the 
last  choice  of  the  local  selectors.  This  was  referred  to  in  the  chapter  on 
"Trunking,  its  Physical  Arrangements  and  Variations." 

The  ringing  is  intermittent,  but  must  be  started  by  the  toll  operator. 
When  the  subscriber  answers,  the  ringing  stops  automatically.  If  the 
subscriber  hangs  up  his  receiver,  the  usual  supervision  is  given,  and  the 
toll  operator  may  start  the  ringing  again,  if  desired. 

Intermittent  ringing  is  used  for  toll  calls  because  it  has  been  found 
to  be  more  effective  than  manual  ringing.  Where  local  ringing  is  periodic 
and  toll  calls  are  rung  by  hand,  much  delay  results.  The  subscriber 
is  inclined  to  think  that  the  manual  ring  is  merely  some  local  call  which 
is  very  shortly  released.  Consequently  he  pays  no  attention  to  it.  But 
the  periodic  signal  gets  his  attention  and  greatly  shortens  the  time  of 
response  to  toll  calls  as  well  as  to  local  calls. 

The  toll  connector  tests  the  called  line  as  does  any  connector.  If  the 
line  is  busy,  it  sends  a  busy  tone  to  the  operator  and  also  extinguishes 
the  supervisory  lamp  at  the  toll  board.  When  the  line  becomes  free, 
the  connector  seizes  the  called  line,  stops  the  busy  tone,  and  lights  the 
supervisory  lamp  at  the  toll  board.  When  the  operator  sees  that  the 
line  is  free,  she  starts  the  ringing  by  pulling  the  ringing  key. 

The  release  of  a  toll-to-local  connection  is  controlled  by  the  toll 


280  AUTOMATIC  TELEPHONY 

operator.  Merely  pulling  the  plug  out  of  the  trunk  jack  causes  the 
switches  to  be  restored  to  normal. 

The  third  method  mentioned  has  been  used  in  but  a  few  plants  in 
which  automatic  switchboards  of  the  old  local  battery  type  are  installed. 
It  has  the  advantage  in  connection  with  automatic  equipment  of  that 
old  type  of  affording  a  better  transmission  circuit  than  could  be  secured 
through  the  automatic  switchboard,  and  of  reducing  the  difficulty  of 
giving  the  operators  adequate  supervision  over  the  long  distance 
connections.  The  equipment  for  this  method  is  so  expensive  to  install 
and  so  much  more  expensive  than  the  other  methods  to  operate  that  it  is 
not  likely  that  it  will  be  used  in  connection  with  modern  automatic 
switchboards  except  under  one  peculiar  condition;  that  is,  where  an 
automatic  switchboard  supersedes  a  manual  switchboard,  and  it  is  not 
thought  advisable  to  replace  the  long  distance  board,  or  to  remodel 
it  in  order  to  adapt  it  for  use  in  handling  connections  directly  through 
the  automatic  switchboard.  Where  this  condition  arises  the  "B"  board, 
or  some  of  the  "A"  operators'  sections  of  the  old  manual  board,  may  be 
used  to  make  up  a  toll  switching  multiple  board  for  setting  up  connections 
in  accordance  with  method  3,  thus  making  it  possible  to  leave  the  old 
long  distance  board  unchanged  and  eliminating  the  expense  of  installing 
special  toll  connectors  on  the  automatic  switchboard. 

Pay-station  Lines. — In  automatic  systems  public  pay  stations  which 
are  used  to  a  considerable  extent  for  long-distance  talking  are  generally 
equipped  with  ordinary  manual  common  battery  telephones  and  three- 
slot  coin  collectors.  Lines  from  pay  stations  of  this  type  run  directly 
to  the  long-distance  board  and,  if  there  are  enough  of  them  to  warrant 
it,  they  all  terminate  in  the  position  of  a  pay-station  operator.  In  any 
event  they  are  multipled  through  the  board  so  that  any  long-distance  line 
operator  has  access  to  them. 

Rural  Lines. — It  has  become  the  practice  to  operate  rural  lines  auto- 
matically much  more  generally  than  formerly.  Great  advantages  have 
thereby  been  secured.  Such  operation  will  be  discussed  in  the  chapter 
on  Rural  Automatic  Telephones. 

Manually  operated  rural  lines  (in  connection  with  automatic  central 
offices)  are  generally  run  to  the  toll  board,  and  where  there  are  enough  of 
them  terminate  on  one  or  more  rural  line  positions. 

Suburban  or  "Rapid  Fire"  Toll  Service. — The  method  for  handling 
calls  from  an  automatic  central  office  to  a  suburban  or  neighboring  office 
when  a  special  fee  is  to  be  charged  for  each  connection  depends  upon 
whether  one  of  the  offices  is  a  manual  office  or  not. 

Calls  from  an  Automatic  Office  to  a  Suburban  Manual  Office. — If  one 
office  is  equipped  with  a  manual  switchboard,  the  preferable  method  is  to 
have  the  automatic  subscribers  connect  to,  and  signal  an  operator  at  the 
manual  switchboard,  by  calling  some  short,  predetermined  and  generally 


LONG  DISTANCE  AND  SUBURBAN  EQUIPMENT  281 

known  number.  For  example,  if  naught  is  used  for  connection  to  the 
regular  long-distance  board,  9  or  91  might  be  used  for  connections  to  the 
manual  office  under  discussion.  At  the  manual  office  these  trunks  may 
terminate  in  regular  subscribers'  line-jack  equipments,  in  front  of  one  or 
more  "A"  operators,  who  will  respond  to  a  subscriber's  signal,  take  his 
order,  make  a  record  of  his  number  and  the  number  of  the  party  desired, 
complete  and  supervise  the  connection  in  the  usual  way;  but  it  is  prefer- 
able to  have  them  terminate  in  cords  and  plugs  in  a  "B"  operator's 
position,  where  they  may  to  better  advantage  receive  the  rather  special 
attention  that  they  deserve. 

It  might  appear  that  since  this  method  makes  it  necessary  for  the 
operator  to  depend  upon  the  calling  subscriber  to  give  her  his  correct 
name  and  telephone  number  in  order  that  the  fee  may  be  charged  to  the 
proper  subscriber's  account,  that  some  subscribers  would  endeavor  to 
secure  free  service  by  giving  the  operator  the  name  and  number  of  some 
other  subscriber.  It  has  been  found,  however,  that  if  a  percentage  of  the 
calls  are  checked,  by  not  putting  them  through  directly,  but  by  telling 
the  calling  party  to  hang  up  and  that  he  will  be  called  when  his  desired 
party  is  secured,  the  knowledge  that  this  may  be  done  at  any  time  almost 
entirely  prevents  cheating. 

Calls  from  the  Manual  to  the  Automatic  Office. — These  may  be 
handled  either  by  means  of  calling  devices  placed  on  the  "A"  operator's 
position  of  the  manual  switchboard,  by  means  of  calling  devices  on  a 
special  "  B"  operator's  position  on  the  manual  switchboard,  or  through  a 
"B"  operators'  switchboard  located  in  the  automatic  office.  A  "B" 
board  in  an  automatic  office  may  be  either  equipped  with  calling  devices, 
or  it  may  be  a  multiple  board  in  which  jacks  are  multipled  with  the 
connector-switch  banks  of  the  automatic  switchboard.  Generally,  the 
most  economical  and  efficient  method  is  to  have  the  calls  set  up  by  the 
operators  in  the  manual  central  office.  If  an  office  is  a  small  one  it  is 
preferable  to  install  a  calling  device  in  each  "A  "  operator's  position.  If 
it  is  a  large  one,  where  the  "A  "  operators  are  worked  at  high  pressure  in 
handling  local  manual  calls,  and  the  number  of  calls  dialed  out  to  the 
automatic  office  is  comparatively  small,  it  is  preferable  to  have  the  calls 
to  the  automatic  office  handled  by  a  special  "B"  operator  to  whom  calling 
manual  subscribers  will  be  switched  by  means  of  transfer  trunks.  The 
"B"  operator  will  respond  to  each  such  call  by  securing  the  number  of 
the  calling  party,  the  number  of  the  automatic  subscriber  desired,  and  the 
name  of  each,  and  will  then  complete  the  connection. 

Suburban  Calls  between  Two  Automatic  Offices.1 — It  should  be  obvi- 
ous that  where  subscribers'  lines  are  equipped  with  measured  service 

1  See  the  paper  entitled  "The  Automatic  Telephone  in  City  Service,"  by  Arthur 
Bessey  Smith,  to  be  found  in  Vol.  II  (pages  1371  to  1378)  of  the  Transactions  oj  the 
American  Institute  of  Electrical  Engineers,  1910. 


282  AUTOMATIC  TELEPHONY 

devices,  such  as  meters  or  coin  collectors  for  registering  or  collecting  a  fee 
for  each  local  call,  that  if  the  same  fee  is  charged  for  the  suburban  call— 
and  it  is  practicable  from  an  engineering  standpoint  to  allow  the  sub- 
scribers to  set  up  their  own  suburban  calls  automatically  (as  it  would  be 
under  almost  any  conceivable  condition)  that  the  registering  or  collecting 
of  fees  for  the  suburban  connections  may  be  done  automatically  without 
the  aid  of  operators.  Where  the  fee  for  the  suburban  call  is  different 
from  that  charged  for  local  service,  or  where  local  service  is  not  furnished 
on  a  measured  plan,  a  switchboard  for  the  use  of  the  operators  required 
may  be  placed  in  either  one  of  the  automatic  offices,  or  a  switchboard 
may  be  placed  in  each  of  them;  that  is,  a  switchboard  may  be  placed  in 
one  office  for  calls  outgoing  from  "A"  office  to  "B"  office  and  a 
switchboard  in  "the  other"  office  may  be  used  for  setting  up  and 
recording  the  calls  outgoing  from  "£"  office  to  "A"  office.  In  either 
event  a  subscriber  desiring  a  suburban  connection  would  secure  the 
operator  by  calling  some  short,  well  understood  number,  as  already  men- 
tioned, and  she  would  complete  the  connection  by  means  of  a  calling 
device.  The  operators'  positions  may  be  equipped  with  cords  and  plugs, 
but  it  speeds  up  the  service  to  have  them  equipped  with  keys  only,  and 
the  arrangements  may  be  such  that  when  a  calling  subscriber  secures  an 
idle  trunk  to  an  operator,  he  thereby  secures  a  corresponding  idle  trunk 
to  the  distant  office  so  that  when  the  operator  responds  to  his  signal  lamp 
and  takes  his  order,  she  may  then  throw  her  calling  device  key,  set  up  the 
balance  of  the  connection  desired  by  the  subscriber  and  restore  her  key 
to  normal.  It  can  be  arranged,  if  desired,  that  when  a  calling  party 
hangs  up,  he  will  release  the  whole  connection  and  at  the  same  time 
furnish  the  operator  the  necessary  supervisory  signal,  showing  the  termi- 
nation of  conversation.  She  has  no  work  to  do  at  the  time  of  disconnec- 
tion, except  to  press  a  key  to  break  the  circuit  through  the  disconnect 
signal  lamp.  The  trunks  will  be  much  more  efficient  if  the  subscribers 
do  the  disconnecting. 

A  Typical  Recording  Trunk  Circuit. — A  connection  from  a  calling  tele- 
phone through  a  lineswitch,  secondary  lineswitch,  first  selector  and  toll 
trunk  repeater  to  a  typical  trunk  circuit  is  shown  in  Fig.  241.  The  cir- 
cuits of  the  primary  lineswitch,  secondary  lineswitch  and  first  selector 
are  the  same  as,  or  very  similar  to,  those  shown  heretofore,  and  will 
therefore  not  be  described.  Attention  is  called,  however,  to  the  tap 
of  the  private  normal  and  release  trunk  of  the  lineswitch  which  is  con- 
nected through  a  condenser  to  the  tone  circuits  of  the  tone  machines 
which  supply  four  distinct  interruptions  or  tones  for  use  on  the  recording 
trunks  to  identify  the  classes  of  service,  as  already  mentioned.  When  a 
tone  is  desired  on  any  line,  a  wire  is  run  from  the  terminal  of  the  bus  bar 
furnishing  the  particular  tone  desired  through  the  condenser  to  the  trunk 
as  shown.  Completion  of  the  circuit  from  this  point  through  the  toll 
recording  operator's  circuit  will  be  described  farther  on. 


LONG  DISTANCE  AND  SUBURBAN  EQUIPMENT  283 

As  already  explained,  the  calling  party  may  secure  connection  to  the 
repeater  of  an  idle  trunk  by  turning  his  dial  from  the  long-distance  finger 
hole,  thus  raising  his  first  selector  shaft  wipers  to  the  naught  bank  level, 
where  they  rotate  until  they  find  an  idle  trunk.  When  connection  to  this 
trunk  is  established  circuit  is  completed  from  either  pole  of  battery 
through  the  windings  of  the  double  wound  line  relay  L.R.  of  the  repeater 
and  the  subscriber's  loop.  L.R.  closes  circuit  from  earth  to  negative 
battery  through  the  1000-ohm  slow  relay  S.R.  and  continues  the  release 
trunk  through  to  the  420-ohm  winding  of  the  double-wound  tone-control 
relay  to  earth.  The  1000-ohm  relay  closes  the  negative  side  of  the  line 
through  to  the  toll  recording  trunk.  As  soon  as  this  occurs  circuit  is 
closed  from  negative  battery  through  the  420-ohm  resistance  coil, 
negative  side  of  the  line,  negative  side  of  the  trunk  and  the  line  relay  A 
of  the  recording  trunk  to  earth.  Relay  A  closes  circuit  through  the  line 
lamp,  signalling  the  recording  operator,  who  responds  by  throwing  her 
key  in  the  direction  to  switch  her  circuit  on  to  the  trunk.  The  tone-test 
circuit  has  already  been  closed  from  tone  terminal  in  lineswitch  unit 
through  condenser,  release  trunk,  release  trunk  of  secondary  lineswitch, 
the  release  trunk  of  the  first  selector,  the  first  selector  private  wiper, 
private  bank  contact,  contact  of  relays  L.R.  and  S.R.  of  the  repeater,  the 
make  before  break  contact  of  the  tone-control  relay  and  the  420-ohm 
winding  of  this  relay  to  earth.  The  tone-control  relay  is  not  operated 
at  this  time  by  the  battery  flow  through  the  B.C.O.  relay  of  the  line- 
switch  via  the  release  trunk.  A  corresponding  current  is  induced  in  the 
22-ohm  winding  of  this  relay  which  transmits  the  tone  to  the  operator 
through  a  circuit  completed  from  earth,  through  the  22-ohm  winding, 
positive  side  of  the  trunk  line,  the  trunk  listening  key,  the  operator's 
head  phone.  At  the  same  time  a  circuit  is  closed  from  earth  through  a 
make  contact  of  operator's  listening  key,  the  cut-off  relay  C  of  the  trunk 
to  negative  battery.  This  relay  breaks  circuit  through  line  relay  A  which 
in  turn  breaks  the  circuit  through  the  line-signal  lamp.  To  cut  off  the 
tone  the  operator  throws  the  key  in  the  opposite  direction,  that  is,  into 
holding  position  and  then  back  to  listening  position.  When  the  key  is  in 
holding  position  a  circuit  is  closed  from  battery  through  the  420-ohm 
resistance  coil  of  the  repeater,  the  negative  side  of  line,  winding  of  500- 
ohm  relay  B  in  the  trunk  circuit,  the  positive  side  of  the  trunk  line,  the 
22-ohm  winding  of  the  tone-control  relay  to  earth.  The  tone-control 
relay  thus  energized  attracts  its  armature  and  shunts  the  tone  circuit 
from  its  420-ohm  winding  through  its  make  contact  to  earth.  Relay  B 
in  the  trunk  circuit  locks  up  through  its  make  contact  and  holds  the  tone 
cut-off  relay  of  the  repeater,  when  the  operator's  key  is  restored  to  the 
listening  position.  Thus  the  tone  is  kept  off  the  talking  circuit  until  the 
calling  subscriber  releases.  If,  after  talking  to  the  calling  subscriber,  the 
recording  operator  should  wish  to  hold  him  on  the  trunk,  but  disconnect 


284 


A  (TOM  A  TIC  TELEPIION  Y 


her  head  phone  while  talking  to  another  party  or  looking  up  some  desired 
information,  she  does  so  by  throwing  the  key  into  holding  position.  This 
leaves  relay  B  across  the  line  and  at  the  same  time  the  key  closes  circuit 
through  the  guard  supervisory  lamp.  The  jack  shown  as  a  part  of  this 
trunk  is  not  essential,  but  is  very  convenient  for  connecting  parties 
through  to  the  chief  operator's  desk  and  for  other  similar  purposes. 

Toll  or  Rural-line  Circuit. — A  typical  toll  or  rural-line  circuit  with  its 
multiple  connection  and  supervisory  signals  is  shown  in  Fig.  242.  The 
ring-up  relay  on  this  circuit  locks  itself  mechanically  when  its  armature  is 
pulled  up,  and  closes  circuit  from  earth  through  the  line  signal  lamp.  At 
the  same  time  it  closes  circuit  from  the  negative  end  of  battery,  through  a 
resistance  and  in  series  through  the  visual  signal  associated  with  the 
multiple  jack  corresponding  to  the  line  in  each  section  of  the  toll  board 
to  earth. 


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Fig.  242. — A  typical  toll  or  rural-line  circuit. 


When  the  operator  responds  by  inserting  the  answering  plug  of  an  idle 
toll  cord  circuit  in  the  line  jack,  a  circuit  is  closed  from  earth  to  which  the 
sleeve  of  the  plug  is  connected  by  the  third  strand  of  the  cord,  through  the 
sleeve  of  the  jack  and  the  cut-off  relay  to  negative  battery.  The  cut-off 
relay  unlocks  the  line  relay  and  at  the  same  time  disconnects  it  from  the 
line.  When  it  does  so  it  closes  a  contact  which  keeps  the  circuit  estab- 
lished from  negative  battery  through  the  visual  signals  guarding  the 
multiples  of  the  line. 

Where  the  rural  lines  are  called  by  the  automatic  subscribers  directly, 
this  line  circuit  and  the  connector  banks  would  be  multiplied  together  by 
connecting  the  two  sides  of  the  line  to  the  terminals  of  the  line  contacts  of 
the  connector  banks,  and  the  sleeve  of  the  jack  to  the  terminal  of  the 


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rcS"'6- 


B      To  Pilot  Relay 


;  tone  signals. 


(Facing  Page  284) 


1 1  Stlf*u*[/atf    * 


Fio.  241.— A  typical  long  distance  call  recording  circuit  equipped  with  discriminating  tone  agnala 


LONG  DISTANCE  AND  SUBURBAN  EQUIPMENT 


285 


private  contact  multiple  of  the  connector  bank.  With  this  arrangement, 
whenever  a  plug  was  inserted  in  the  jack,  the  earth  connection  that  pulled 
up  the  cut-off  relay  would  put  a  guarding  potential  on  the  corresponding 
private  multiple  of  the  bank,  and  vice  versa,  whenever  a  connector  switch 
was  connected  to  this  line  the  guarding  earth  potential  established  on  the 
private  bank  contact  and  its  multiple  would  pull  up  the  cut-off  relay  of 
the  line,  which  would  close  the  circuit  through  the  visual  busy  signals. 

Toll  Cord  Circuit. — The  toll  cord  circuit  used  with  this  line  is  shown  in 
Fig.  243.  The  plug  of  this  circuit,  which  is  marked  "Toll,"  must  always 
be  used  in  toll  line  jacks,  regardless  of  whether  the  operator  is  answering 
or  calling;  and  the  plug  marked  "Auto"  must  always  be  used  in  the  trunks 
to  the   automatic   switchboard   or  in   pay-station  lines.     When   a  line 


To20-*/n- 
•Ji  su  la  ting 
Trans. 


Make  cutoff  Relay  1000 uj  for 
Toll  Lines,  overflow  Trunks,  and 
_  Ring  up  or  reverse  battery 

ToSivKey  supervision.  Make  cutoff  Relay 
Mon.  Ckt    4(5qw  for  Ton  Trunks  and 

common  battery  supervision 


ToSw.Key 
CD.  Ckt 


To  Stv  Key    — . 
List  Ckt      p# 


To  Mas  ter 
Ring  Key 


Fig.  243.- 


-A  typical  cord  circuit  for  connecting  toll  lines  to  local  automatic  switchboard 

lines. 


operator  has  received  an  order  for  a  connection  to  one  of  her  toll  lines, 
she  picks  up  the  toll  plug  of  an  idle  cord  circuit  and  inserts  it  into  the 
jack  of  the  line  on  which  the  desired  party  is  to  be  called.  As  already 
mentioned,  current  immediately  flows  from  earth  through  the  third 
conductor  of  the  cord,  the  sleeve  of  the  plug,  sleeve  of  the  jack,  and  the 
cut-off  relay  of  the  line  which  cuts  off  the  ring-up  relay  and  leaves  the 
line  clear  for  the  operator  to  ring.  It  will  be  noted  that  when  she  throws 
her  ringing  key  the  tip  and  ring  of  the  plug  are  disconnected  from  the 
cord  circuit  and  connected  directly  to  the  ringing  generator  bus  bars. 
She  then  restores  her  ringing  key  to  normal,  throws  her  listening  key  and 
awaits  the  answer  of  the  operator  at  the  distant  end  of  the  line. 

Should  the  operator  at  the  other  end  desire  at  any  time  to  secure  the 
operator  at  the  local  end,  she  does  so  by  sending  generator  current  over 


28G  AUTOMATIC  TELEPHONY 

the  lino  which  causes  the  3100-ohm  ring-up  relay  to  attract  momentarily 
its  armature  and  open  the  shunt  around  the  200-ohm  supervisory  relay 
to  earth.  The  supervisory  relay  immediately  acts  and  closes  the  circuit 
through  the  lamp,  giving  the  required  signal  to  the  operator.  When  the 
operator  responds,  by  throwing  her  listening  key,  it  closes  a  circuit  to 
earth,  which  again  shunts  out  the  supervisory  relay  causing  it  to  release 
its  armature  and  break  the  circuit  through  the  lamp. 

The  automatic  end  of  this  cord  circuit  is  separated  from  the  toll  end 
by  a  condenser  of  4  m.f.  capacity  inserted  in  each  side  of  the  cord.  Super- 
vision in  the  automatic  end  is  provided  by  two  relays,  A  and  B,  operated 
through  the  third  strand  of  the  cord,  the  sleeve  of  the  plug  and  the  sleeve 
of  the  jack,  and  by  the  double  wound  polarized  relay  which  sometimes  is 
bridged  across  the  line;  but  when  relay  A  is  energized  sufficiently  to 
attract  its  armature,  one  winding  of  the  polarized  relay  is  connected  to 
earth  and  the  other  winding  is  connected  to  negative  battery  through  the 
30-ohm  German  silver  resistance  coil  which  is  wound,  for  convenience, 
on  the  spool  of  relay  B.  The  purpose  of  this  coil  is  to  prevent  danger 
from  accidental  short-circuiting  of  the  two  poles  of  the  battery  through 
the  springs  of  relay  A,  by  inserting  the  30-ohm  resistance  between  the 
springs  and  the  negative  pole  of  the  battery. 

The  automatic  end  of  this  cord  circuit  is  designed  so  that  it  may  be 
used  with  at  least  four  different  types  of  circuits:  First,  with  common 
battery  pay-station  lines;  second  with  overflow  trunks;  third,  with  regular 
toll  service  trunks  terminating  in  special  connector  switches  and,  fourth, 
with  local  battery,  magneto  signalling,  pay-station  lines. 

Before  describing  in  detail  the  various  circuits  with  which  this  cord 
circuit  is  used,  an  indication  will  be  given  of  the  method  by  which  it 
differentiates  between  the  various  circuits. 

Relay  "A  "  (Fig.  243)  will  not  pull  up  in  series  with  1000  ohms  on  46- 
volt  battery,  but  will  pull  up  through  400  ohms.  Therefore,  on  the 
circuits  where  it  is  desired  to  have  the  polarized  relay  remain  bridged 
across  the  line  (overflow  trunks  and  local  battery  pay  stations),  the 
sleeve  relay  of  the  line  circuit  has  a  resistance  of  1000  ohms  or  more;  but 
on  the  service  trunks  and  common  battery  pay-station  lines,  where  it  is 
desired  that  the  polarized  relay  supply  talking  current  to  the  line,  relay 
A  is  energized  through  a  400-ohm  sleeve  relay.  Supervision  is  therefore 
obtained  as  explained  in  the  following  descriptions  of  the  respective  line 
and  trunk  circuits. 

Rural-line  Switchboard  Incoming  Trunk  Circuit. — An  incoming  trunk 
circuit  for  use  in  receiving  orders  for  rural-line  connections  from  auto- 
matic subscribers  is  shown  in  Fig.  244.  This  circuit  terminates  in  a  jack 
in  the  rural-line  position  and  in  selector  banks  on  the  automatic  switch- 
board. As  previously  mentioned,  it  is  common  practice  to  arrange  the 
numbering  and  trunking  plan  so  that  automatic  subscribers  may  place 


LONG  DISTANCE  AND  SUBURBAN  EQUIPMENT 


287 


orders  for  rural-line  connections  by  calling  not  to  exceed  two  digits. 
When  a  two-digit  number  is  used  in  a  multi-office  system,  the  first  digit 
is  used  in  operating  a  first  selector  in  the  automatic  subscriber's  own 
office,  which  selects  an  idle  trunk  to  the  office  in  which  the  rural-line 
board  is  located.  The  second  digit  is  used  in  operating  a  second  selector 
switch  in  that  office  to  extend  the  subscriber's  connection  over  an  idle 
trunk  circuit,  similar  to  that  shown  in  the  above  diagram,  to  the  rural-line 
board.  As  soon  as  the  second  selector  switch  has  completed  the  connec- 
tion, current  flows  from  earth  through  one  winding  of  the  double-wound 
relay  of  the  line  circuit,  through  the  calling  subscriber's  loop;  back 
through  the  other  winding  of  the  line  relay  to  negative  battery.  This 
relay  closes  a  circuit  from  earth  to  the  release  trunk  P  of  the  second  selec- 


Fig.  244.- 


-Circuit  of  trunk  incoming  to  a  rural-line  switchboard  from  an  automatic 
switchboard. 


tor.  It  also  closes  circuit  from  earth  through  the  line  lamp,  signalling 
the  operator,  and  circuit  from  earth  through  the  1000-ohm  slow-acting 
relay  A  to  negative  battery.  Relay  A  closes  circuit  from  earth  through 
relay  B  to  negative  battery.  In  case  the  operator  does  not  respond 
promptly,  or  a  subscriber  should  wish  to  attract  her  attention  at  any 
time,  he  may  flash  the  line  lamp  by  moving  his  receiver  switch  hook  up 
and  down  rapidly,  or  by  making  another  turn  of  his  dial.  This  will  cause 
the  double-wound  line  relay  to  make  and  break  the  circuit  through  the 
lamp,  but  the  slow-acting  relay  A  will  keep  the  release  trunk  connected 
to  earth,  preventing  the  second  selector  switch  from  releasing. 

When  the  operator  answers,  using  the  automatic  plug  of  the  cord 
shown  in  Fig.  243,  the  400-ohm  cut-off  relay,  C.O.R.,  of  the  trunk  is 
energized  in  series  with  relay  A  of  the  cord  circuit  and  breaks  the  circuit 


288 


AUTOMATIC  TELEPHONY 


through  the  line  lamp.  The  operator  throws  the  listening  key  in  her  cord 
circuit,  and  after  ascertaining  the  rural-line  party  desired  by  the  auto- 
matic subscriber  used,  inserts  the  toll  plug  in  the  jack  of  the  proper  rural 
line,  circuit  of  which  is  similar  to  that  shown  in  Fig.  242,  and  sends  out 
the  proper  signal.  The  supervisory  features  have  already  been  explained. 
When  the  calling  automatic  subscriber  disconnects  before  the  operator 
withdraws  her  plug  from  the  trunk  jack,  the  cut-off  relay  closes  circuit 
from  earth  through  the  back  contact  of  the  slow-acting  relay  B,  to  the 
release  trunk  so  soon  as  the  armature  of  B  falls  back.  Since  the  circuit 
through  B  is  not  broken,  however,  until  the  armature  of  the  slow-acting 
relay  A  falls  back,  B  does  not  complete  the  circuit  to  the  release  trunk 
until  the  selector  switches  have  had  time  to  release  The  purpose  of 
connecting  the  release  trunk  to  earth  after  the  switches  have  released,  is 
to  prevent  the  trunk  being  seized  by  another  subscriber  before  the  op- 
erator has  withdrawn  the  plug  from  the  jack. 


Dist  Frame 


,§      '■■Through  of  her  Vjsuals  to  Ground 


Res.  Coil 


Fig.  245. — An   overflow    trunk   from    a    toll    switchboard    to    an    automatic  switchboard. 


Since  the  circuits,  of  chief  operator's  and  monitor's  desks,  through 
switching  toll  cord  circuits,  toll  test  panels,  etc.,  may  be  the  same  as  any 
efficient  circuits  used  in  manual  practice,  space  will  not  be  occupied  by 
a  description  of  them. 

Overflow  Trunks. — One  end  of  an  overflow  trunk  is  shown  in  Fig. 
245.  The  other  end  terminates  in  selectors  and  reversing  battery  con- 
nectors, the  circuits  of  which  have  been  explained  in  a  previous  chapter. 
When  the  automatic  plug  of  the  toll  cord  circuit  is  inserted  in  the  jack  of 
an  overflow  trunk,  the  third  strand  of  the  cord  is  connected  to  negative 
battery,  through  the  1300-ohm  busy-control  relay  of  the  trunk.  Relay 
A  of  the  cord  circuit  therefore  does  not  operate  but  the  busy-control  relay 
of  the  trunk  does  so,  closes  the  positive  side  of  the  trunk,  and  operates 
the  visual  busy  signal  associated  with  this  trunk,  and  with  each  of  its 
multiples  in  the  various  positions.     When  the  operator  throws  her  call- 


LONG  DISTANCE  AND  SUBURBAN  EQUIPMENT  289 

ing-device  key  to  call  the  desired  automatic  party,  she  breaks  the  shunt 
which  has  been  maintained",  through  back  contact  of  relay  B,  back  con- 
tact of  the  polarized  relay  and  the  springs  of  the  calling-device  key  to 
earth  and  which  has  kept  relay  B  from  operating,  so  that  relay  B  is 
energized  by  a  circuit  from  earth  through  its  own  winding,  winding  of 
relay  A,  third  strand  of  the  cord  and  the  1300-ohm  busy  control  relay  of 
the  trunk  to  negative  battery.  When  it  operates,  it  closes  the  circuit 
from  the  trunk  supervisory  lamp  of  the  cord  circuit  through  the  back 
contact  of  the  polarized  relay  to  the  springs  of  the  calling-device  key, 
so  that  so  soon  as  the  calling-device  key  is  restored  to  normal  the  super- 
visory lamp  circuit  is  completed  to  earth  and  the  lamp  glows.  When  the 
automatic  subscriber  responds,  the  direction  of  the  current  through  the 
polarized  relay  is  reversed  by  the  connector  switch  used  and  it  swings  its 
armature  away  from  normal  position  thus  breaking  the  circuit  through 
the  lamp.  Relay  B,  however,  remains  energized  during  conversation, 
and  when  a  subscriber  replaces  his  receiver  on  the  switch-hook,  thus 
reversing  the  direction  of  current  flow  back  to  normal  through  the  polar- 
ized relay,  the  armature  of  which  swings  back  to  normal  position,  the 
circuit  through  the  supervisory  lamp  is  again  immediately  established 
giving  the  operator  the  disconnect  signal. 

Local  Battery  Magneto  Signalling  Pay  Station  or  Line. — The  circuit 
of  a  local  battery  magneto  signalling  pay  station  or  line  for  use  with  this 
cord  circuit  should  be  practically  the  same  as  the  toll  and  farmer  line 
circuit  (Fig.  242)  already  described.  As  the  diagram  indicates,  this 
circuit  is  provided  with  a  1000-ohm  cut-off  relay,  so  that  relay  A  of  the 
cord  circuit  will  not  be  operated  when  the  plug  enters  the  jack  and  conse- 
quently the  polarized  relay  will  be  left  bridged  across  the  line  to  act  as  a 
ring-up  relay  for  supervisory  purposes. 

Pay-station  Lines. — On  a  common  battery  pay  station  line  (Fig. 
246),  when  a  plug  is  inserted  in  the  jack,  current  flows  from  negative 
battery  through  the  400-ohm  cut-off  relay,  sleeve  of  the  jack,  sleeve  of 
the  plug,  third  strand  of  the  cord  and  the  winding  of  relay  A  to  ground. 
Relay  A  attracts  its  armature  switching  one  winding  of  the  polarized 
relay  to  earth  and  the  other  winding  to  negative  battery.  A  closes 
circuit  also  through  the  supervisory  signal  lamp.  At  the  same  time,  the 
cut-off  relay  operates,  disconnecting  earth  from  the  positive  side  of  the 
line  and  disconnecting  the  500-ohm  line  signal  relay  from  the. negative 
side  of  the  line.  It  also  closes  a  circuit  from  negative  battery  through 
a  resistance  coil  to  the  visual,  busy  signal  associated  with  the  jack  of  this 
line  and  in  series  through  each  of  its  multiples  in  the  various  sections 
to  earth;  so  that  a  guarding  signal  shows  wherever  a  multiple  of  this 
line  appears. 

The  operator  rings  on  the  line  by  throwing  her  ringing  key,  which  she 
then  restores  to  normal.     When  the  pay-station  party  responds,  current 

19 


290 


A I '  TOM  A  TIC  TELEl'IION  Y 


flows  from  earth  through  one  winding  of  the  polarized  relay  and  the 
subscriber's  loop,  back  through  the  other  winding  of  the  polarized  relay 
to  negative  battery.  The  polarized  relay  swings  its  armature  away 
from  normal  and  in  doing  so  breaks  the  circuit  through  the  supervisory 
lamp  of  the  cord. 

When  the  subscriber  finishes  conversation  and  places  his  receiver  on 
the  switch-hook,  the  armature  of  the  polarized  relay  returns  to  normal 
position  and  the  supervisory  lamp  lights,  giving  the  operator  the  required 
disconnect  signal. 

Toll  Service  Trunk  Circuits. — In  modern  automatic  service  it  is 
customary  to  divide  the  connectors  which  serve  a  certain  group  of  sub- 
scribers into  two  classes,  regular  connectors  and  combination  toll  and 
regular  connectors.  The  regular  connectors  are  seizeable  only  by  sub- 
scribers in  the  ordinary  course  of  their  call.     The  combination  connectors 


Fig.  246. — A  pay-station  line  equipment  at  its  toll-board  end. 

are  also  seizeable  by  subscribers,  but  the  trunking  arrangement  is  such 
that  no  combination  connector  of  a  group  will  be  seized  by  a  subscriber 
until  after  all  of  the  regular  connectors  in  the  group  are  in  use.  When 
thus  seized,  the  combination  connectors  give  a  service  identical  with  that 
of  a  regular  connector. 

The  trunks  to  the  combination  connectors,  in  addition  to  appearing 
in  the  bank  of  the  regular  third  selectors,  are  also  multipled  to  the  banks 
of  the  toll  third  selectors.  When  a  toll  operator  dials  a  certain  number 
the  toll  third  selector  will  route  the  call  to  one  of  the  combination  con- 
nectors serving  the  group  of  which  that  particular  number  forms  a  part. 
The  combination  connector  will  now  function  as  a  toll  connector,  the 
principal  difference  being  that  the  starting  of  the  automatic  ringing  will 
now  be  under  the  control  of  the  toll  operator.  Should  the  toll  operator 
call  a  busy  line,  the  combination  connector  will  advise  her  of  the  fact, 


LONG  DISTANCE  AND  SUBURBAN  EQUIPMENT 


291 


and  as  soon  as  the  line  becomes  idle,  will  indicate  to  her  that  she  may 
now  depress  her  ringing  key. 

A  combination  connector  when  giving  either  kind  of  service  will,  of 
course,  busy  itself  both  in  the  regular  third  selector  banks  and  in  the 
toll  third  selector  banks. 

In  Fig.  247  is  shown  diagrammatically  the  various  circuits  used  in 
extending  a  call  from  the  toll  board  to  an  automatic  subscriber.  At  A 
is  represented  any  standard  toll  cord  circuit  with  equipment  added 
such  that  a  dial  may  be  cut  in  series  with  the  sleeve  of  the  calling  plug. 
At  B  is  represented  an  intermediate  selector  circuit.  In  this  switch  the 
impulsing  relay  is  controlled  over  a  third  conductor.  The  talking  circuit 
is  cut  straight  through,  and  a  fourth  conduct  or  forms  the  customary  release 
trunk.  At  C  is  shown  a  final  selector,  or  as  commonly  called  "a  toll 
third  selector."  Here  the  talking  circuit  is  carried  through  a  repeating 
coil  and  it  is  this  switch  which  feeds  talking  battery  to  the  called  sub- 
scriber. The  impulsing  relay  is  here  controlled  over  a  fourth  conductor, 
the  usual  release  trunk  forming  the  third  conductor.     At  D  is  shown  a 


Toll  Cord 
Ckt. 


Intermediate 
Selector 


Banks  of 
Final      Local 3rdszz^ 
Selector  J_ 


Calling 
Device' 


Combination 
Regular  a  Toll 
Connector 


Fig.  247 


Operating  Conductor 
-Toll  to  local  circuits  in  skeleton. 


combination  toll  and  regular  connector.  The  usual  three-wire  trunk 
is  carried  to  the  banks  of  the  regular  third  selectors.  The  talking  con- 
ductors are  cut  straight  through  for  toll  service.  The  third  conductor 
is  the  usual  release  trunk  for  toll  service  and  the  impulsing  relay  is  con- 
trolled over  the  fourth  conductor. 

Toll-cord,  Sleeve  Dialing. — This  is  shown  in  somewhat  skeleton  form 
in  Fig.  248.  There  is  enough  to  show  the  essentials  of  sleeve  dialing 
and  supervision  over  the  loop  of  the  line  wires.  Relay  B  is  for  ring-down 
supervision  from  a  toll  line  when  the  latter  is  manually  operated.  Relay 
A  is  for  supervision  from  the  local  automatic  switchboard.  The  sleeve 
conductor  is  grounded.  When  it  is  desired  to  dial  toward  the  automatic 
switchboard  (local)  the  operator  throws  the  calling-device  key  to  "local" 
which  operates  the  springs  marked  "CD-L."  This  cuts  off  the  toll  end 
of  the  cord  and  switches  the  calling  device  CD  into  the  sleeve  of  the 
local  plug. 

Toll  Selector,  with  Operating  Trunk. — The  circuit  of  a  toll  selector 
which  may  serve  as  first,  second,  etc.,  is  shown  in  Fig.  249.     Here  it  is 


292 


A  UTOMA  TIC  TELEPHON  Y 


shown  with  a  jack  on  the  toll  board.  The  insertion  of  the  plug  by  the 
operator  closes  the  sleeve  circuit  to  the  operating  relay  OpRy  and  operates 
the  visual  busy  signal.  The  operating  relay  in  the  selector  acts  on  the 
magnets  and  other  relays  exactly  as  was  described  in  connection  with 
regular  selectors.     When  the  selector  cuts  through  to  the  next  switch 

Operator's  Telephone 
Talk  Monitor 


Local 


Fig.  248. — Toll  to  local  cord  circuit,  sleeve  dialing. 

ahead,  the  operating  wire  is  switched  from  the  operating  relay  in  this 
switch  to  that  of  the  next  switch.  The  release  trunk  is  carried  through 
separately. 

Toll  Third  Selector,  with  Repeating  Coil.— The  toll  third  selector, 
as  it  is  called,  has  a  circuit  like  that  shown  in  Fig.  250.     Whether  it  is  a 


Visual 
dust/ 


Jprckon 
Toll      r=,      Ji—i- 
board  Q 


Fig.  249.— Toll  selector. 


real  "third"  selector  or  not  depends  upon  the  size  of  the  exchange.     In 
any  case  this  is  the  switch  which  feeds  battery  to  the  called  telephone. 
This  switch  functions  as  an  ordinary  selector,  except  that  the  oper- 
ating relay  OpRy  takes  the  place  of  the  usual  line  relay  and  is  controlled 


LONG  DISTANCE  AND  SUBURBAN  EQUIPMENT 


293 


over  the  operating  trunk,  P-l.     The  talking  circuit,  indicated  by  the 
heavy  lines,  is  entirely  separate  from  the  rest  of  the  selector. 

When  a  connection  has  been  extended  to  this  switch,  the  operating 
relay  OpRy  is  energized  and  in  turn  pulls  up  the  release  relay  RlRy.  The 
release  relay  grounds  the  release  trunk  P-2  so  as  to  hold  the  switches 
back  of  it  in  the  operated  position,  opens  the  release  circuit  to  the  release 
magnet  Rise,  and  prepares  the  circuit  to  the  vertical  magnet  VM . 


Fig.  250. — Toll  third  selector  (transmission  selector). 

At  each  interruption  of  the  calling  device  in  the  sleeve  of  the  toll  cord, 
the  line  relay  drops  back  and  operates  the  vertical  magnet  in  series  with 
the  series  relay  SeRy  through  a  back  contact  on  the  switching  relay  SwRy, 
and  a  front  contact  on  the  release  relay. 

The  off-normal  springs  close  on  the  first  vertical  step,  preparing  the 
release  circuit  and  closing  a  circuit  for  the  interrupter  relay  IR  through 
the  front  contact  of  the  series  relay  SeRy  to  release  trunk  ground.  The 
interrupter  relay  pulls  up  and  locks  itself  to  the  ground  at  the  operating 
relay  (front  contact)  which  at  the  end  of  the  group  of  impulses  will  be 
steady.     The  interrupter  relay  prepares  a  circuit  for  the  rotary  magnet. 


294  AUTOMATIC  TELEPHONY 

When  the  impulses  cease,  the  operating  relay  comes  to  rest  energized. 
The  series  relay  drops  back  and  closes  the  circuit  of  the  rotary  magnet  to 
release  trunk  ground.  The  rotary  magnet  pulls  the  wipers  onto  the  first 
trunk  in  the  level  and  near  the  end  of  its  stroke  opens  the  circuit  of  the 
interrupter  relay  so  that  the  latter  falls  back. 

As  the  interrupter  relay  falls  back,  it  opens  the  circuit  of  the  rotary 
magnet,  causing  it  to  fall  back  and  close  the  interrupter-relay  circuit. 
If  there  is  a  ground  on  the  private  wiper  P-2,  the  switching  relay  SivRy 
will  not  be  able  to  act  and  the  interrupter  relay  will  pull  up  again  and 
cause  the  rotary  magnet  to  move  the  wipers  to  the  next  trunk.  When  an 
idle  trunk  is  found,  no  ground  on  the  private  contact,  the  switching  relay 
will  pull  up,  preventing  by  its  resistance  any  further  action  of  the  inter- 
rupter relay. 

The  pulling  up  of  the  switching  relay  SwRy  has  several  effects.  It 
closes  the  circuit  of  the  front  contact  of  the  operating  relay  OpRy  through 
P-l  to  the  toll  connector  so  as  to  pull  up  its  operating  relay.  It  switches 
the  private  wiper  P-2  from  the  interrupter  circuit  to  a  200-ohm  resistance 
and  negative  battery  by  way  of  a  back  contact  on  relay  H.  It  opens  the 
impulsing  circuit  of  the  back  contact  of  the  operating  relay,  because  the 
latter  is  now  to  act  as  repeater  to  the  connector.  It  also  closes  the  two 
terminals  of  the  back-bridge  relay  BBR  through  the  repeating  coil  to  the 
talking  wipers  -\-W  and  —W. 

A  brief  examination  of  the  repeating  coil  and  its  connections  will  show 
that  there  is  a  ringing  relay  which  is  responsive  to  ringing  current  from 
the  toll  board.  There  is  also  battery  on  the  two-line  wires  through  200 
ohms  to  each  side,  and  this  is  removable  by  the  pulling  up  of  the  back- 
bridge  relay  through  relay  F-l.  Thus  the  called  station  may  give  super- 
vision. As  soon  as  the  connection  from  the  toll  board  gets  this  far,  this 
battery  operates  the  relay  A  bridged  across  the  toll  cord  and  lights  the 
supervisory  lamp. 

Combined  Toll  and  Regular  Connector. — We  pass  now  to  the  con- 
nector circuit  which  is  shown  in  Fig.  251.  It  may  on  occasion  be  seized 
by  a  local  selector,  in  which  case  the  call  arrives  on  the  three  wires  shown 
in  the  lower  left-hand  corner.  In  that  case  it  will  function  like  the  other 
connectors.  But  the  call  which  we  are  considering  will  come  in  over  the 
four  wires  marked  "toll  selector  banks." 

This  circuit  is  that  of  a  multi-level  group  connector  such  as  is  used  for 
subscribers  who  have  more  than  ten  lines  in  one  group.  It  will  illustrate 
the  action  as  well  as  the  ordinary  connector. 

The  seizure  of  this  connector  by  the  toll  third  selector  pulls  up  the  line 
relay  LR  by  its  negative  winding  alone,  in  series  with  relay  J.  The 
dead  wire  prevents  noise. 

The  line  relay  pulls  up  the  release  relay  RlRy  which  grounds  the 
release  trunk  P-2  in  addition  to  its  other  duties.     The  release  trunk  has 


LONG  DISTANCE  AND  SUBURBAN  EQUIPMENT 


295 


relay  J  in  it,  which  assures  relay  K  of  remaining  energized.  It  also  closes 
the  positive  line  as  relay  K  closes  the  negative  line. 

The  vertical  action  is  as  usual  in  switches,  the  series  relay  preparing 
things  for  the  next  function.  The  interrupter  relay  IR  prepares  the 
rotary-magnet  circuit  for  use  at  the  first  idle  level. 

At  the  end  of  the  group  of  impulses,  the  vertical  wiper  VW  takes 
charge  of  the  vertical  magnet  and  causes  it  to  select  an  idle  level.  The 
vertical  wiper  passes  over  a  vertical  bank  which  is  made  busy  by  a  ground 
through  resistance  if  all  the  lines  in  a  level  are  occupied.     As  long  as  the 


P-l  Opcr 


P-  2  fflse 


Toll 
Set 

Hanks 


Local 

Sol 

Banks 


-dust/ Tone 


■~-tV 


Fig.  251. — Toll  and  local  connector. 


vertical  wiper  finds  ground,  the  vertical  relay  VRy  is  energized  and  the 
vertical  magnet  caused  to  vibrate  alternately  with  the  interrupter  relay 
(instead  of  the  rotary  magnet) .  When  the  vertical  wiper  finds  no  ground, 
the  vertical  relay  falls  back  and  switches  the  vertical  magnet  out  and  the 
rotary  magnet  in.  After  this,  the  interrupter  relay  and  the  rotary 
magnet  vibrate  each  other  until  an  idle  line  is  found. 

When  the  private  wiper  PW  has  found  the  idle  line,  no  ground,  the 
switching  relay  SwRy  pulls  up  and  cuts  the  lines  through  to  the  line 
wipers,  +W  and  —W.  It  also  grounds  the  private  wiper  and  operates 
the  ringing-interrupter  start  Rilntr Start.  The  latter  controls  the  inter- 
ruption of  the  ringing  generator  as  has  been  described  before.     The 


296  AUTOMATIC  TELEPHONY 

switching  relay  also  inserts  150  ohms  in  the  vertical  magnet  circuit  and 
switches  release  relay  ground  from  the  right-hand  contacts  of  the  series 
relay  to  a  back  contact  on  the  same,  which  thus  reaches  the  release  trunk. 

Now  the  toll  operator  rings  on  the  trunk  with  alternating  current. 
(Fig.  248.)  This  operates  the  ringing  relay  RiRy  in  the  toll  third  selector 
(Fig.  250)  which  pulls  up  relay  H  of  the  same  switch.  Relay  H  locks 
itself  through  the  back  contact  of  the  ring  cut-off  relay  RiCOR  to  the 
release  trunk.  Relay  H  opens  the  wiper  side  of  the  repeating  coil, 
switches  the  negative  line  from  the  back-bridge  relay  BBR  to  the  winding 
of  the  ring  cut-off  relay  and  negative  battery,  and  cuts  off  the  battery 
connection  of  the  release  trunk  which  leads  to  the  connector. 

In  the  connector  (Fig.  250)  this  last  action  causes  relay  J  to  fall  back 
and  connect  the  plus  wiper  +  W  to  the  interrupted  generator  and  ground. 
The  ringing  thus  starts  to  operate  just  as  was  described  in  connection 
with  regular  connectors  for  local  traffic.  The  ringing  will  continue  until 
the  called  station  answers  or  the  toll  operator  pulls  down  the  connection. 
It  requires  only  a  short  ring  to  start  it. 

When  the  called  station  answers,  the  telephone  set  draws  current 
through  the  ring  cut-off  relay  RiCOR  of  the  third  selector  (Fig.  250) 
which  pulls  up  and  releases  the  relay  H.  The  latter  falls  back,  restoring 
battery  to  the  release  trunk  to  the  connector  (Fig.  251)  so  that  relay  J 
pulls  up  and  cuts  off  the  generator. 

The  called  station  now  draws  battery  current  through  the  back- 
bridge  relay  BBR  of  the  third  selector.  (Fig.  250.)  This  relay  pulls  up 
and  energizes  relays  F-l  and  F-2. 

Relay  F-l  disconnects  the  ringing  relay  RiRy  so  that  the  toll  operator 
can  not  ring  as  long  as  the  called  station  has  the  receiver  off  the  hook, 
and  cuts  battery  and  ground  off  the  trunk  to  the  toll  board.  This 
permits  the  relay  A  (Fig.  248)  to  fall  back  and  put  out  the  local  super- 
visory lamp. 

After  this,  the  called  station  signals  the  toll  operator  by  the  hook  and 
the  toll  operator  controls  the  switches. 

To  release  the  connection,  the  toll  operator  pulls  cut  the  plug  from 
the  jack.  (See  Figs.  248  and  249.)  This  releases  the  operating  relay  in  the 
third  selector  (Fig.  250)  which  in  turn  lets  the  operating  relay  in  the 
connector  (Fig.  251)  fall  back.  Each  of  these  releases  its  part  of  the 
connection. 

If  the  called  line  had  been  busy,  the  vertical  wiper  would  have  found 
no  contact  free  from  ground.  The  first  level  of  the  group  would  in  this 
condition  have  dead  ground  on  it,  which  will  operate  the  marginal  busy 
relay  ByRy.  The  latter  pulls  up,  cutting  off  the  vertical  magnet  and 
placing  a  busy  tone  on  the  plus  line,  so  that  the  toll  operator  will  hear 
the  tone  and  know  the  conditions. 

If  she  leaves  the  connection  up,  and  one  of  the  lines  in  the  group  should 


LONG  DISTANCE  AND  SUBURBAN  EQUIPMENT  297 

become  free,  the  busy  relay  ByRy  will  fall  back  and  permit  the  vertical 
magnet  to  hunt  for  the  idle  level  and  the  rotary  magnet  to  rotate  the 
wipers  to  the  idle  line  in  that  level.  This  action  of  waiting  is  called 
"camping  on  busy." 

Combined  Connector  Used  as  Local  Connector. — If  the  connector  of 
Fig.  251  is  seized  by  a  local  selector,  the  connection  comes  in  on  the  three 
wires  at  the  lower  left-hand  corner.  The  line  relay  LR  is  pulled  up,  and 
it  operates  the  release  relay  as  usual.  The  latter  grounds  the  release 
trunk  from  the  local  banks  and  the  release  trunk  P-2  from  the  toll  selec- 
tors, the  latter  through  the  20-ohm  relay  J.  Relay  J  will  not  operate  at 
any  time  under  this  seizure,  hence  the  lines  to  the  toll  selectors  remain 
open.  Relay  K  also  remains  inactive,  so  that  the  200-ohm  winding  of 
the  ring  cut-off  relay  RiCOR  is  ready  to  be  used. 

The  action  of  dialing  and  selecting  an  idle  line  out  of  the  group  is  the 
same  as  was  described.  When  it  comes  time  to  ring,  it  rings  automat- 
ically, because  the  interrupted  generator  is  connected  through  the  back 
contact  of  relay  J  and  the  trip  winding  of  the  ring  cut-off  relay  is  con- 
nected through  the  back  contact  of  relay  K. 

When  seized  by  a  local  selector,  the  connector  will  not  "camp  on 
busy"  for  the  contact  of  the  busy  relay  ByRy  is  short-circuited  by  the 
back  contact  of  relay  K.  The  wipers  will  run  up  to  the  last  level,  then 
will  rotate  to  the  last  line  and  off  the  bank,  when  the  cam  springs  marked 
"  11-R"  operate,  cutting  the  overflow  tone  into  series  with  the  line  relay 
and  pulling  up  the  ring  cut-off  relay.     The  subscriber  must  release. 

Long  Distance  Automatic  Calling. — As  already  stated,  long  distance 
lines  of  moderate  length,  terminating  at  one  end  in  a  central  office  in 
which  an  automatic  switchboard  is  installed,  may  be  made  considerably 
more  efficient  by  connecting  them  up  so  that  an  operator  at  the  distant 
end  of  the  line  may  use  a  dial  to  call  directly  any  subscriber  to  the  auto- 
matic central  office. 

It  has  been  found  that  the  manual  switching  of  toll  lines,  among 
themselves  and  to  subscriber  lines,  entails  considerable  loss  of  time.  This 
is  chiefly  due  to  the  time  required  to  gain  the  attention  of  operators  who 
have  other  lines  to  serve  besides  the  one  in  question.  It  lengthens  the  time 
required  to  set  up  a  connection,  it  interferes  with  supervision,  and  delays 
the  release  of  the  circuits  after  both  subscribers  have  hung  up  their 
receivers.  This  is  so  great  a  drag  on  the  service  that  it  has  led  to  the 
establishment  of  many  direct  toll  lines  between  important  points,  so  as 
to  do  away  with  intermediate  switching.  This  lowers  the  time  efficiency 
of  the  long  direct  line,  because  it  can  not  be  used  for  any  intermediate 
traffic. 

For  at  least  thirteen  years  (1920)  toll  lines  have  been  switched  by 
automatic  means  and  experimenters  have  been  at  work  producing  appa- 
ratus which  is  adapted  to  this  service.     The  first  discovery  made  was  that 


L".IS 


AVTOMATH  '   TELEPHONY 


automatic  switching  increased  the  capacity  of  the  toll  line  at  least  50  to 
100  per  cent.  This  remarkable  increase  is  due  to  the  speed  of  connection, 
the  accuracy  and  promptness  of  supervision,  and  the  quick  clearing  of  the 
line  after  both  subscribers  have  hung  up  their  receivers. 

No  matter  how  the  switches  are  controlled,  the  line  must  permit 
manual  operation  at  any  time,  must  operate  automatical^  in  both  direc- 
tions if  both  ends  are  at  automatic  exchanges,  when  seized  must  give 
positive  indication  to  all  stations  concerned,  and  must  give  supervision 
of  both  calling  and  called  subscriber  to  the  one  operator  who  controls 
the  connection. 

Uniform  operation  is  highly  desirable.  This  point  has  received  atten- 
tion in  developing  automatic  switching  over  toll  lines.  It  is  so  arranged 
that  if  the  operator  plugs  into  the  jack  of  an  automatic  toll  line  and  rings, 
the  call  will  go  through  manually,  as  formerly.     But  if,  after  plugging 


Trunks  from  .    .         •■    r    .  „.n 

Selector        Automatic  Exchange-  A 

Banks-:. 


f : 


Local 
Conn,. 


Trunks  from 
Automatic  Exchange  =-B    Selector 
Banks 


k-ToII  Conn. 


Toll-*1- 

Selectors 

I®  |  Jack. 


Toll  Line  Circuit -1 


Toll  Line  Circuit-? 


Toll  Board  Toll  Board 

Fig.  252. — Automatic  toll  lines  (scheme). 

into  the  jack,  the  operator  throws  the  calling  device  key,  she  can  put  the 
call  through  automatically.  After  dialing  the  number,  the  operator 
rings  with  the  regular  ringing  key.  This  operation  holds  no  matter 
whether  she  is  switching  toll  lines  only,  or  if  she  automatically  completes 
the  connection  to  the  called  subscriber. 

Inter-city  Scheme. — For  the  sake  of  simplicity,  this  is  illustrated  by 
two  small  exchanges,  for  the  principles  do  not  depend  on  the  size.  The 
toll  lines  (Fig.  252)  terminate  at  each  end  in  toll  selectors,  and  in  jacks 
and  lamps  on  the  toll  board.  The  trunks  from  local  selector  banks  also 
run  to  the  toll  board. 

When  a  subscriber  in  B  desires  to  call  a  subscriber  in  A,  he  dials  the 
toll  operator  according  to  the  number  given  in  the  directory.  This  gives 
him  a  trunk  terminating  in  a  lamp  and  jack  as  shown  at  the  right  in  the 
illustration.  The  toll  operator  answers  in  the  usual  way.  She  has  a 
calling  device  which  may  be  associated  with  any  pair  of  cords.     When 


LONG  DISTANCE  AND  SUBURBAN  EQUIPMENT 


299 


she  plugs  into  the  jack  of  the  toll  line,  she  makes  it  busy  at  both  ends, 
showing  the  busy  lamp  at  exchange  A,  so  that  the  toll  operator  there  will 
not  try  to  use  that  line.  The  operator  at  B  then  dials  the  call  through  the 
toll  selectors  to  the  subscriber  desired. 

The  operator  at  B  controls  the  connection  and  gives  it  the  usual 
supervision.  When  the  subscribers  are  through,  she  pulls  out  the  switch- 
board plug,  which  causes  the  switches  to  release. 

The  identity  of  the  calling  station  may  be  verified  by  having  the 
subscriber  hang  up  his  receiver.  When  the  connection  is  ready,  the 
operator  calls  the  originating  subscriber  automatically  and  permits  con- 
versation. 

Toll  Switching  Office. — If  between  two  such  exchanges  there  is  a  toll 
central  office,  the  lines  can  be  interconnected  by  automatic  switches,  so 

Toll  Lines 


\\  IY 


X     %'      Nj 


N 


Automatic 
Switches 


T^^T^xsTV^ 


T^J^^S" 


T^T^T^" 


MulHple 
VToll 
Board 


Lamp 
Fig.  253. — Toll  switching  office  (scheme). 


as  to  save  the  time  of  manual  switching.  (See  Fig.  253.)  Consider  four 
toll  lines  meeting  at  a  toll  office,  which  is  manually  and  automatically 
operated.  The  former  may  be  needed  for  handling  traffic  to  and  from 
local  subscribers. 

Each  toll  line  terminates  in  three  places,  the  toll  board,  the  automatic 
switchboard  multiple,  and  the  jack  of  the  automatic  switch  which 
belongs  to  that  line. 

If  a  distant  exchange  seizes  a  toll  line,  it  is  automatically  made  busy 
on  the  banks  of  the  toll  selectors  and  the  busy  lamps  belonging  to  it  on 
the  toll  board  are  lighted.  The  distant  exchange  then  dials  the  number 
of  the  toll  line  desired.  When  that  line  is  seized,  it  is  made  busy  on  the 
selector  banks  and  toll  board. 


300 


A  V  TOM  A  TIC  TELEPIION  Y 


Toll-line  Controlling  Circuit. — A  toll-line  controlling  circuit  which  is 
very  much  used  is  shown  in  its  simplest  form  in  Fig.  254.  There  are 
two  line  relays,  LR-l  and  LR-2,  both  normally  grounded.  There  is  also 
a  controlling  relay  at  each  end  of  the  line,  CR-1  and  CR-2. 

If  the  line  is  seized  at  the  left  end,  the  control  relay  CR-1  is  energized 
and  held  so  during  occupancy.  It  switches  the  line  relay  from  ground 
to  battery  and  calling  device  (or  the  equivalent).  This  causes  current 
to  flow  in  the  line  and  in  both  line  relays.  By  suitable  means,  not  shown, 
LR-l  at  the  sending  end  is  not  permitted  to  act  on  local  automatic 
switches.  LR-2  pulls  up  and  closes  the  circuit  of  an  automatic  switch 
leading  into  the  exchange.     This  seizes  the  line. 

When  the  calling  person  (operator  or  subscriber)  dials  the  call  number 
LR-2  vibrates  and  repeats  the  number  to  the  exchange  apparatus  at  the 
right,  and  sets  up  the  connection. 


Line 


To  Automatic 
Switch 


CR-1 


L.R.-2X> 


To  Automatic 
Switch 


c.D.-i  C£=:: 

± 


'Irfem^Mg 


r?co 


CR.-2 


-2 


Fig.  254. — Toll  line  controlling  circuit. 


Release  is  accomplished  by  restoring  relay  CR-1  to  normal.  This 
switches  the  line  back  to  ground,  the  current  fails,  LR-2  falls  back,  and 
releases  the  connection  in  the  distant  exchange  as  usual. 

Simplex  Calling. — Figure  255  shows  the  above  described  simple  con- 
trolling circuit  applied  to  a  simplex  circuit.  The  elements  are  so  simple 
that  further  description  is  not  necessary. 

Figure  256  shows  this  circuit  as  it  is  used  for  one  way  calling  between 
several  cities  in  the  state  of  Indiana. 

The  automatic  switchboard  is  located  at  A.  B  is  a  switching  station 
45  miles  from  A,  and  C  is  a  large  manual  plant  45  miles  from  B,  and  90 
miles  from  A.  B  and  C  are  both  provided  with  calling  devices  for  doing 
automatic  calling  into  A .  Between  A  and  B  is  another  toll  station,  of  one 
telephone  only,  which  is  bridged  across  the  line  in  the  usual  way  for 
manual  calling  and  is  not  shown  in  the  diagram.  Each  calling  device 
(indicated  by  CD)  is  connected  between  earth  and  the  middle  point  of  a 
repeating  coil,  bridged  across  the  line,  following  the  practice  used  in 
connecting  up  simplex  telegraph  circuits  to  telephone  lines. 


LONG  DISTANCE  AND  SUBURBAN  EQUIPMENT 


301 


When  B  wishes  to  call  some  subscriber  at  A  she  throws  her  calling 
key  and  operates  her  calling  device  in  the  usual  way.  The  calling  device 
opens  and  closes  the  circuit  between  the  two  sides  of  the  line  and  ground 
thus  operating  the  Morse  telegraph  relay  connected  between  the  line  and 
battery  at  A.  This  relay  makes  and  breaks  the  circuit  to  the  two-wire 
automatic  switchboards  at  A  so  that  the  automatic  switches  set  up  the 
number  desired.  All  adjusting  of  impulses  due  to  line  leakages  is  readily 
taken  care  of  by  the  switchboard  attendant  at  A,  by  adjusting  the  Morse 
relay  to  suit  the  amount  of  leakage  on  the  line  at  the  time. 

When  C  wishes  to  call  a  party  at  A,  she  throws  her  calling-device  key 
and  if  the  line  from  B  to  A  is  not  busy,  C's  visual  signal  will  operate  and 
thus  indicate  that  the  line  is  clear,  but  if  the  line  from  B  to  A  is  in  use  the 
visual  signal  circuit  will  be  open  in  one  of  two  places. 


To  Manual 
and  Automatic 
Switchboards 


Fig.  255. — Simplex  controlling  circuit. 


1.  If  B  is  calling  A,  the  simplex  tie  at  B  will  be  open  at  B's  calling  key. 

2.  If  A  has  called  B,  or  if  B  has  called  A  manually,  so  that  there  is  a 
plug  inserted  in  the  line  jack  at  A,  the  ring  of  the  jack  will  be  connected 
to  earth  through  the  third  conductor  of  the  cord  circuit  and  consequently 
the  "manual  cut-off  relay"  will  be  operated  opening  the  connection 
between  each  side  of  the  line  and  the  repeating  coil  installed  in  the  circuit 
to  the  automatic  switchboard. 

If  C  finds  the  line  from  B  to  A  in  use,  she  leaves  her  calling-device  key 
in  operated  position.  This  does  not  interfere  with  operations  between 
B  and  A,  and  as  soon  as  that  section  of  the  line  is  clear  the  visual  at  C 
operates. 

At  the  same  time  the  through-switching  relay  at  B  operates,  cutting 
off  both  of  the  taps  at  B,  switching  both  sides  of  the  line  straight  through 
to  A,  and  bridging  out  B's  calling-device  key  so  that  B  can  not  break  the 
connection  from  B  to  A  by  throwing  her  calling-device  key.  It  should  be 
noted  that  if  such  a  station  as  B  should  be  connected  in  through  a  cable 


302 


A  l !  TOM  A  TIC  TELEPHON  Y 


LONG  DISTANCE  AND  SUBURBAN  EQUIPMENT 


303 


of  considerable  length,  that  a  transmission  loss  in  the  through  circuit  can 
be  eliminated  by  installing  the  through-switching  relay  and  repeating 
coil  at  B  on  the  pole  where  the  cable  connects  to  the  through  line. 

When  C's  visual  signal  indicates  that  the  line  is  clear,  she  operates 
her  calling  device  in  the  usual  way  and  the  Morse  relay  at  A  operates  the 
automatic  switches  as  already  described. 

Whenever  C  or  B  calls  automatically  into  A,  the  "automatic  cut-off 
relay"  at  A  operates,  due  to  the  grounding  of  the  release  trunk  of  the 
automatic  switches,  and  cuts  off  the  manual  cut-off  relay,  while  at  the 
same  time  it  closes  the  circuit  through  the  visual  signal  at  A,  which 
operates  and  indicates  that  the  line  is  in  use. 


High 


lOtY 


Fig.  257. — Quadruplex  dialing  on  toll  lines. 

On  leaky  lines  better  operation  can  be  secured  following  telegraph 
practice  by  installing  a  battery  at  the  sending  end  of  the  line  and  con- 
necting the  Morse  relay  at  the  receiving  end  to  earth.  With  this  arrange- 
ment a  leak  on  the  line  will  shut  off  some  of  the  current  intended  for  the 
Morse  relay— a  difficulty  which  can  generally  be  overcome  by  adjusting 
the  relay  to  suit  the  weakened  current  reaching  it;  but  with  the  battery 
at  the  receiving  end,  a  moderate  leak  makes  the  relay  act  sluggishly  and 
a  greater  leak  renders  it  inoperative. 

Quadruplex  Dialing. — The  principles  of  quadruplex  telegraphy  have 
been  applied  to  automatic  dialing  over  toll  lines.  (See  Fig.  257.)  A  few 
simple  changes  were  necessary  to  adapt  it  to  this  purpose,  but  they  do 
not  affect  the  fundamentals  of  quadruplex. 

Two  physical  toll  lines  are  shown,  No.  1  and  No.  2,  and  a  phantom 
circuit  superimposed  on  them.     The  quadruplex  apparatus  is  attached  to 


304  AUTOMATIC  TELEPHONY 

the  center  of  the  phantom,  in  simplex  fashion.  There  is  the  usual  neutral 
relay,  with  its  "bug  trap"  relays  R-Q  and  R-7,  and  the  artificial  line 
AL.  Relay  R-Q  controls  the  automatic  line  by  relays  R-4  and  R-5. 
The  polar  relay  controls  another  automatic  line  by  relays  similar  to 
them. 

The  various  currents  are  controlled  by  a  group  of  relays  at  the  right. 
R-8  is  common — it  switches  the  line  relays  (neutral  and  polar)  from 
ground  to  battery.  R-9  switches  from  low  voltage  battery  to  high  volt- 
age.    Relays  R-10  and  #-11  switch  from  negative  to  positive  polarity. 

If  an  operator  plugs  into  the  jack  of  line  No.  1,  the  cut-off  relay  COR 
will  cut  off  the  line  drop  LD  but  the  dialing  relay  R-l  will  not  pull  up, 
because  of  a  condenser  in  the  bridge  across  the  cord.  The  operator 
can  ring  with   ordinary  A.C.  and  throw  the  drop  at  the  distant  city. 

If,  after  plugging  into  the  jack,  the  operator  cuts  out  the  condenser 
in  the  cord  bridge,  relay  R-l  will  cause  R-8  and  R-9  to  pull  up,  sending 
high  voltage  current  onto  the  simplex  circuit.  The  line  relays  at  the 
sending  end  will  not  respond,  but  the  neutral  relay  at  the  distant  city 
will  pull  up.  It  will  operate  its  slow  relay  R-7  which  will  energize 
R-Q  in  series  with  resistance.  R-Q  energizes  R-4  and  R-5  in  series.  The 
former  closes  the  line  to  the  automatic  exchange,  while  the  latter  grounds 
a  wire  while  pulls  up  relay  R-2.  The  latter  R-2  switches  the  toll  line 
No.  1  to  the  automatic  line  and  lights  the  busy  lamp  on  the  manual  toll 
board. 

When  the  operator  at  the  calling  end  of  the  line  dials,  she  controls 
her  relay  R-l,  which  makes  R-9  vibrate,  lowering  and  raising  the  current, 
to  which  the  neutral  relay  in  the  distant  city  responds.  The  latter  short- 
circuits  R-Q  every  time  that  it  falls  back,  but  R-7  remains  energized. 
R-4  follows  the  impulses  and  repeats  them  into  the  automatic  line,  setting 
up  the  connection.  During  this  time  R-3  short-circuits  the  impedance 
coil. 

Release  is  accomplished  by  pulling  the  plug  out  of  the  jack.  All  the 
relays  fall  back,  taking  all  current  off  the  line,  and  releasing  the  automatic 
connection. 

The  polar  side  of  the  quadruplex  controls  the  other  automatic  line  in 
a  very  similar  way.  No  bug  trap  is  necessary.  The  calling  operator 
controls  relays  R-8,  R-10,  and  R-ll  in  series,  the  first  switching  the  line 
relays  as  before  and  remaining  energized  during  the  holding  time,  the 
second  and  third  reversing  the  polarity  so  as  to  operate  the  polar  relay 
in  the  distant  city. 

The  phantom  circuit  may  be  used  manually,  or  one  side  of  the  quad- 
ruplex may  be  used  for  it  instead  of  for  one  of  the  physical  circuits. 

Composite  Dialing. — The  principles  of  composite  telegraphy  have 
been  applied  to  automatic  dialing  over  toll  lines.  Figure  258  shows  the 
essential  principles.     It  is  not  necessary  to  give  here  a  description  of  the 


LONG  DISTANCE  AND  SUBURBAN  EQUIPMENT 


305 


laws  upon  which  composite  rests.     The  retardation  coils  and  condensers 
are  arranged  in  the  way  which  telegraphy  has  found  is  best. 

Each  Morse  leg  has  been  brought  out  and  treated  exactly  as  shown  in 
the  simple  controlling  circuit  of  Fig.  254.     There  is  a  normally  grounded 


C.Q-2 


Retard''^  « 

'con  yi^i 


L.R-4 


jo Composite 

m  Rmger 

CRr4M  hc.R.^i 


Fig.  258. — Toll  line  with  composite  dialing. 

circuit  from  LR-1  over  one  line  wire  to  LR-2,  and  another  from  LR-3  to 
LR-4.     Each  may  be  used  separately  without  regard  to  the  other. 

Figure  259  shows  the  composite  arranged  to  be  called  either  manually 
or  automatically.     In  the  upper  left-hand  corner  is  the  manual  toll  board 


Busy©^ 


Fig.  259. — Composite  dialing  circuits. 

apparatus.  At  the  left  center  are  the  selector  banks  by  means  of  which 
the  toll  line  can  be  called  automatically.  This  is  used  if  this  is  a  toll 
switching  office  as  described  under  the  head  "Toll  Switching  Office." 
In  the  upper  right-hand  corner  is  the  toll  line  with  the  composite  ringing 

20 


306  AUTOMATIC  TELEPHONY 

apparatus.  The  rest  of  the  apparatus  is  a  selector  which  has  access  to 
the  automatic  exchange  or  may  form  part  of  it. 

The  toll-cord  circuit  used  with  this  apparatus  is  arranged  for  sleeve 
dialing — the  calling  device  operates  to  interrupt  the  sleeve  circuit. 
This  leaves  the  line  circuit  free  for  talking  and  supervision. 

If  the  operator  plugs  into  the  jack  and  rings,  the  distant  city  will  be 
signalled  by  the  line  drop  LD  as  usual,  because  the  line  drop  can  not  be 
cut  off  except  by  the  operator  at  that  end  of  the  line.  But  the  mere  act 
of  plugging  into  the  jack  makes  the  line  busy  at  the  other  end,  by  the 
lighting  of  the  busy  lamp. 

The  insertion  of  the  plug  into  the  jack  sends  current  through  the 
sleeve  and  operates  the  relay  R-2.  It  pulls  up  relay  R-4,  which  operates 
the  cut-off  relay  COR,  cuts  ground  off  the  Morse  leg,  and  grounds  the  busy 
wire.  The  latter  lights  the  busy  lamp  and  grounds  the  private  bank 
contact  P-2  on  the  selectors,  to  prevent  calling  from  that  source.  The 
relay  R-2  also  switches  the  Morse  leg  to  battery  (through  a  small  resist- 
ance) so  that  current  now  flows  through  the  line  relay  LR  over  the  toll- 
line  wire  to  the  distant  city  where  it  passes  through  a  similar  line  relay 
to  ground.  The  line  relay  LR  in  the  calling  city  can  not  do  anything, 
because  relay  R-4  has  taken  away  its  ground. 

The  line  relay  LR  in  the  distant  city  pulls  up  and  operates  P-3  which 
in  turn  pulls  up  R-5.  P-3  short-circuits  the  condenser  in  the  repeating 
coil,  and  prepares  a  ground  for  the  circuit  to  private  wiper  P-\.  Relay 
R-5  opens  the  release  magnet  circuit,  grounds  the  busy  wire,  and  prepares 
the  impulsing  circuit  to  the  vertical  magnet. 

When  the  operator  in  the  calling  city  dials  over  the  sleeve,  she  causes 
R-2  to  vibrate.  R-4  remains  energized.  The  impulsing  of  the  line 
current  causes  the  line  relay  LR  in  the  distant  city  to  vibrate,  sending 
impulses  to  the  vertical  magnet  of  its  selector.  The  off-normal  springs 
ONS  close,  also  the  series  relay  SeRy  causes  the  interrupter  relay  IR  to 
lock  up  and  prepare  the  rotary  magnet  for  action. 

When  the  group  of  impulses  ceases,  the  line  relay  LR  comes  to  rest 
energized.  Soon  the  series  relay  SeRy  falls  back,  upon  which  the  rotary 
magnet  rotates  the  wiper  shaft  and  wipers  onto  the  level  selected.  The 
rotary  magnet  RM  and  the  interrupter  relay  cause  each  other  to  vibrate 
as  long  as  there  is  a  ground  on  the  private  wiper  P-2.  When  an  idle 
trunk  is  reached,  the  interrupter  relay  can  not  act,  the  switching  relay 
SwRy  is  able  to  pull  up  and  the  rotation  stops.  The  switching  relay  cuts 
off  the  sleeve  circuit  through  R-2,  also  the  vertical-magnet  circuit,  cuts 
the  busy  wire  through  to  the  selector  ahead,  connects  the  lines  to  the  line 
wipers,  and  closes  the  circuit  of  private  wiper  P-l. 

Further  dialing  is  repeated  by  the  line  relay  LR  and  relay  R-3  by  way 
of  the  fourth  wire  through  P-l.  At  the  same  time  relay  P-6  operates 
relay  P-l  to  cut  out  the  repeating  coil  and  cut  the  talking  lines  through. 


LONG  DISTANCE  AND  SUBURBAN  EQUIPMENT 


307 


Release  is  accomplished  by  pulling  the  plug  out  of  the  jack,  which 
restores  R-2  at  the  sending  end  to  normal,  cuts  off  the  line  current,  and 
de-energizes  the  line  relay  LR  at  the  distant  end.     Relays  R-3  and  R-5 


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Switch  through  Columbus  Automatically 
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Fig.  260. — Map    showing   long    distance   automatic    lines    centering   at  Columbus,  Ohio. 


follow,  closing  the  release  magnet  circuit  which  releases  the  selector  as 
usual. 

If  the  call  comes  to  the  toll  line  through  a  selector  instead  of  through 
the  manual  jack,  private  contact  P-2  acts  as  the  regular  release  trunk 


308  AUTOMATIC  TELEPHONY 

and  lights  the  busy  lamp  as  soon  as  the  selector  seizes  the  trunk.  Private 
contact  P-l  is  the  carrier  of  the  impulsing  circuit,  taking  the  place  of  the 
sleeve  of  the  cord.  As  soon  as  R-2  pulls  up,  it  causes  RA  to  ground  the 
busy  wire  and  contact  P-2.  The  rest  of  the  action  is  as  was  described 
for  manual  calling. 

Automatic  Long  Distance  Calling  into  Columbus,  O. — Figure  260  is  a 
skeleton  map  of  the  state  of  Ohio,  showing  the  various  towns  and  cities 
which  use  automatically-operated  toll  lines  into  the  Columbus,  Ohio, 
automatic  system.  The  longest  line  is  one  to  Cleveland,  which  is  145 
miles  distant  from  Columbus.  While  on  this  map  but  one  line  is  indicated 
from  each  outlying  point  to  the  capital,  in  reality  a  number  of  these  cities 
are  connected  to  Columbus  by  several  automatic  toll  lines.  For  example 
Dayton  is  connected  to  Columbus  by  three  lines. 

Automatic  Through  Switching.— All  of  these  lines  terminating  in  the 
Columbus  automatic  switchboard  have  numbers  by  which  they  can  be 
called  automatically  by  any  other  toll  station  equipped  with  a  calling 
device  in  the  long  distance  line  system.  The  lines  to  Cleveland  are  num- 
bered 023  and  024,  and  a  toll-board  operator  in  Dayton,  for  example, 
can  dial  either  of  these  numbers  and  secure  a  through  connection  to  Cleve- 
land, provided  the  line  called  is  not  busy.  If  it  is  busy  the  Indianapolis 
operator  is  automatically  given  a  busy  signal.  Several  of  the  large 
hotels  in  Columbus  have  in  their  lobbies  switchboards  connected  into 
this  long-distance  system  and  equipped  with  dials,  so  that  the  attendants 
call  the  various  cities  in  the  system  without  the  aid  of  the  operators  at 
the  toll  board  in  the  Columbus  Central  Office.  All  lines  are  connected 
into  the  Columbus,  manually-operated,  toll  board,  so  that  they  may  be 
switched  either  manually  or  automatically. 

Note. — It  is  suggested  that  the  student  of  long  distance  automatic  calling  refer 
to  the  very  interesting  paper  discussing  some  other  phases  of  the  subject  which  was 
presented  before  the  American  Institute  of  Electrical  Engineers  by  Messrs.  H.  M. 
Friendly  and  A.  E.  Burns,  printed  in  the  Proceedings  for  July,  1912;  and  Arthur 
Bessey  Smith,  American  Institute  of  Electrical  Engineers,  Dec.  12,  1919. 


CHAPTER  XIII 
RURAL  AUTOMATIC  TELEPHONES 

Telephone  service  on  rural  lines  differs  from  that  in  cities  and  towns 
in  that  dwellings  are  much  farther  apart,  there  is  usually  no  common 
center  except  the  nearest  town,  and  greater  dependence  is  placed  on  the 
telephone.  If  we  consider  a  larger  area,  there  are  several  small  towns 
which,  with  their  surrounding  country,  may  be  considered  as  rural,  and 
the  arrangements  for  telephoning  made  to  include  all. 

Manual  service  in  small  exchanges  is  necessarily  poorer  than  in 
large  exchanges,  and  the  personality  of  the  operator  more  easily  felt. 
Even  under  the  most  favorable  conditions  of  long  training  of  operators 
and  close  supervision  of  them,  large  exchanges  suffer  from  delays  and 
inaccuracies.     Small  exchanges  are  even  less  well  served. 

Almost  all  communities  require  continuous  service  and  are  entitled 
to  it.  The  value  of  night  service  is  out  of  proportion  to  its  volume. 
Such  telephone  calls  as  are  made  during  the  night  are  usually  the  result 
of  necessity  and  deserve  as  prompt,  accurate  attention  as  can  possibly 
be  given. 

It  has  been  the  experience  of  companies  who  operate  automatic 
exchanges  that  before  long  the  farmers  begin  to  ask  that  they,  too,  be 
given  automatic  service,  and  in  some  cases  demand  it.  This  has 
resulted  in  a  large  growth  of  rural  automatic  telephones  in  the  past  years, 
so  that  today  (1920)  they  form  a  considerable  factor. 

Viewed  from  a  broad  standpoint,  these  rural  services  may  be  classified 
as  follows: 

Individual  line,  the  case  of  the  prosperous  farmer. 

Party  line,  connected  to  an  automatic  exchange  in  town. 

Isolated  community  automatic  exchange. 

String  of  small  exchanges. 

Radial  or  satellite  system  of  small  exchanges. 

Network  of  small  exchanges. 

Individual  Rural  Line. — In  general,  the  conditions  are  the  same  as  for 
an  individual  line  in  town,  except  in  those  cases  in  which  the  line  is  so 
long  that  common  battery  can  not  be  worked  for  talking.  The  range  of 
common  battery  transmission  has  been  extended  by  the  bringing  out  of 
a  sensitive  transmitter  for  weak  current.  But  beyond  this,  it  is  some- 
times necessary  to  use  local  battery  for  talking.     (See  Fig.  261.) 

Rural  Party  Line. — The  method  of  signalling  used  is  the  factor  which 
is  most  important  in  arranging  the  party  line.  Both  selective  ringing 
and  code  ringing  are  in  use. 

309 


310 


A  U  TOM  A  TIC  TELEPIION  Y 


The  placing  of  many  telephones  on  one  line,  especially  in  rural  regions, 
increases  the  chance  that  a  subscriber  will  interfere  with  the  dialing  of 
another.  If,  while  impulses  are  being  sent  by  a  telephone,  another 
telephone  comes  across  the  line,  it  short-circuits  the  dialing  instrument 
and  causes  the  call  to  fail.  This  is  obviated  by  providing  each  telephone 
with  a  hook  stop,  which  permits  the  hook  to  rise  part  way,  only  far 
enough  to  connect  the  receiver  across  the  line  in  series  with  a  2  m.f. 
condenser.  It  enables  the  subscriber  to  listen  without  interfering  with 
whatever  dialing  may  be  going  on.  If  the  line  proves  to  be  free,  the 
subscriber  presses  the  hook  release,  the  lever  then  rises  to  the  full  extent 
of  its  stroke,  connecting  the  calling  device.  Besides  preventing  inter- 
ference with  dialing,  this  divice  permits  the  rural  subscriber  to  listen 
on  the  line — a  habit  which  is  firmly  established  and  warmly  defended  by 
the  subscribers. 


Fig.  261. — Local  battery  automatic  telephone  circuit. 


Rural  lines  are  often  served  by  the  regular  lineswitch  boards  if  there  is 
nothing  special  to  their  operation.  But  if  the  number  of  such  lines  is 
considerable,  or  the  conditions  of  their  operation  differ  greatly  from  those 
of  other  lines,  they  will  be  handled  by  lineswitch  units  devoted  to  that 
service  alone.  The  special  apparatus  required  is  localized  to  this  part  of 
the  equipment. 

It  frequently  occurs  that  rural  lines  produce  so  much  traffic  that  it 
overloads  the  group  of  first  selectors  belonging  to  the  lineswitch  board. 
This  is  relieved  either  by  subdividing  the  lineswitches  into  smaller  groups, 
or  by  providing  each  rural  line  with  its  own  first  selector  instead  of  a 
lineswitch.  In  this  case  the  selector  performs  the  functions  of  repeater 
as  well  as  of  selector.  The  relative  cost  of  the  two  plans  varies  with 
conditions. 


RURAL  AUTOMATIC  TELEPHONES  311 

Because  of  the  number  of  bells  and  condensers  (sometimes  ten) 
bridged  across  the  line,  and  the  lower  insulation  which  is  at  times  un- 
avoidable, the  control  circuit  and  apparatus  must  have  a  greater  factor 
of  safety  than  is  usual.  To  secure  this  repeater  is  interposed  between 
the  rural  line  and  the  regular  exchange  apparatus.  This  repeater 
has  two  line  relays  (Fig.  262),  one  in  each  battery  lead.  The  impulsing 
is  done  with  the  relay  which  is  connected  to  the  positive  grounded  battery 
terminal.  Also,  during  the  time  that  impulses  are  coming  in,  the  imped- 
ance of  the  negative  line  relay  is  greatly  lowered.  These  two  provisions 
greatly  increase  the  reliability  of  selection. 

The  line  requirements  are  as  follows : 

,     Maximum  safe  loop  resistance 1,000  ohms. 

Minimum  safe  insulation  resistance,  wire  to  wire 
or  wire  to  earth 25,000  ohms. 

If,  however,  there  is  a  separate  repeater  or  a  selector-repeater  for 
each  line,  so  that  its  line  relay  can  be  adjusted  for  that  line  alone,  the 
loop  resistance  can  be  greatly  increased. 


Fig.  262. — Rural  line  controlling  circuit. 

Code  Ringing. — Code  ringing  is  very  largely  employed  in  manual 
exchanges  over  large  areas  in  the  United  States,  is  understood  by  the 
subscribers,  and  seems  to  be  satisfactory. 

Code  ringing  permits  all  the  bells  to  be  alike  and  to  be  bridged  across 
the  line.  In  order  not  to  interfere  with  dialing,  the  bells  are  wound  to 
3600  ohms  (approximately  5000  ohms  impedance  to  ten  cycles  per 
second)  and  the  condensers  limited  to  about  0.2  m.f.  Signals  are  made 
by  combinations  of  long  and  short  rings. 

Under  certain  conditions  push-button  ringing  seems  to  be  desirable. 
By  this  plan,  each  telephone  is  equipped  with  a  push-button  which 
grounds  the  negative  line  wire  and  operates  a  ringing  relay  in  the  switch 
equipment.  The  code  signals  are  sharper  and  more  easily  understood 
than  when  hand  generators  are  used,  but  not  as  good  as  if  made  by  a  code- 
ringing  machine  in  the  central  office.  The  grounding  of  the  negative  line 
and  opening  of  the  positive  line  causes  the  positive-line  relay  to  fall  back, 
while  the  negative-line  relay  remains  energized  and  holds  the  switches. 
The  positive  relay  controls  the  ringing  indirectly. 

Reverting  calls  are  made  by  dialing  the  regular  call  number  as  usual. 


312 


AUTOMATIC  TELEPHONY 


The  subscriber  then  presses  the  push-button  for  the  code  of  signals, 
which  are  sent  out  on  the  line. 

Simplex  Dialing. — Simplex  dialing  on  rural  lines  is  accomplished  by 
the  plan  outlined  in  Fig.  263.  The  line  is  equipped  with  local  battery 
magneto  ringing  telephones.  The  center  of  each  bell  has  a  tap  taken  out 
and  run  through  the  switch-hook  so  that  the  calling  device  CD  may  be 
connnected  to  ground.  The  line  runs  through  a  repeating  coil  in  the 
central  office,  the  center  of  which  is  connected  by  a  line  relay  to  negative 


L:>    To  Automatic 
'-    Switchboard 


Fig.  263. — Simplex  controlling  circuit  for  rural  party  line. 

battery.  The  line  relay  controls  the  loop-dialing  circuit  to  the  auto- 
matic switches. 

When  the  receiver  is  taken  from  the  hook,  the  line  relay  pulls  up 
and  closes  the  line  to  the  automatic  switchboard.  The  operation  of  the 
calling  device  is  thus  relayed  to  the  switches,  and  the  connection  estab- 
lished in  an  obvious  manner.  Figure  264  shows  the  complete  telephone 
circuit. 

Reverting  calls  on  this  line  are  rung  in  the  usual  manner  by  hand 
generators.  It  is  possible  to  add  this  equipment  to  existing  magneto 
telephones  of  suitable  type 


Fig.  264. — Rural  telephone,  magneto,  simplex  dialing. 

Code  Ringing  Machine. — There  is  now  in  use  a  code-ringing  machine 
which  is  part  of  the  central  office  equipment  which  is  capable  of  giving 
better  signals  than  can  be  made  by  hand.  As  many  as  twenty  codes 
are  available,  although  good  service  from  the  traffic  standpoint  will  not 
permit  more  than  ten  stations  on  one  line,  unless  the  conditions  are 
unusual.     In  many  instances  the  number  should  be  much  less  than  ten. 

The  subscriber  dials  the  call  number  in  the  usual  way  and  waits. 
The  code  machine  begins  to  ring  long  and  short  rings  according  to  the 


RURAL  AUTOMATIC  TELEPHONES 


313 


number  selected,  and  stops  when  the  called  station  answers  or  the  calling 
subscriber  hangs  up  his  receiver. 

Reverting  calls  are  made  by  dialing  a  reverting  call  number,  posted 
on  the  telephones  of  the  line,  and  hanging  up  the  receiver.  All  the  bells 
on  the  line  will  ring  according  to  the  code.  When  the  called  station 
answers,  the  ringing  stops.  If  the  called  party  does  not  answer,  the  call- 
ing subscriber  stops  the  ringing  by  taking  down  his  receiver  for  a  moment 
and  replacing  it. 

Selective  Ringing. — Selective  ringing  is  in  some  cases  better  than 
code  ringing  and  may  even  be  requested  by  subscribers. 

Ten  bells  can  be  rung  selectively  on  one  line  by  bridging  five  from 
each  line  to  earth  and  using  the  five  frequencies  of  the  "multiple  har- 
monic" or  the  " non-multiple  harmonic."  A  telephone  circuit  arranged 
for  this  service  is  shown  in  Fig.  265.  The  bell  is  permanently  bridged 
from  the  required  side  of  the  line  to  earth  and  is  not  cut  off  by  the  switch- 
hook. 

Reverting  calls  are  handled  in  the  same  way  as  was  described  for  city 
lines.     By  calling  a  special  number  connection  is  secured  to  a  reverting 


Pol.  Pec. 

Ml 


2M.F. 


rtx 


\lc 


'ih-OO- 


.here 


+L 


Fig.  265. — Rural  telephone  circuit 
10-party  line. 


Fig.  266. — Isolated  exchange. 


call  switch,  which  rings  alternately  the  bell  of  the  calling  and  of  the  called 
subscriber. 

Isolated  Exchange. — The  isolated  exchange  is  relatively  rare.  It  has 
no  connections  to  other  exchanges.  The  switchboard  is  very  simple, 
because  there  is  no  in  and  out  traffic,  which  is  the  source  of  most  of  the 
complications.  Figure  266  shows  the  relations,  both  town  and  rural 
party  lines  being  served  by  the  same  small  automatic  board,  which  is 
termed  "Community  Automatic  Exchange." 

String  of  Exchanges. — A  string  of  exchanges  is  found  very  often  in 
new  or  sparsely  settled  country,  or  in  more  settled  country  where  the 
topography  causes  this  relation  of  interests.  There  may  be  but  one  toll 
line,  following  a  line  of  railroad.  In  such  a  case  (Fig.  267)  a  community 
automatic  exchange  is  installed  in  each  small  town,  with  individual  lines 
to  town  subscribers  and  party  lines  reaching  out  into  the  country. 

The  toll  office  and  miscellaneous  services  (information,  complaint, 
etc.)  are  located  wherever  they  can  receive  the  best  attention.  It  often 
pays  to  have  all  the  toll  switching  for  several  exchanges  done  by  one  of 


314 


A  UTOMA  TIC  TELEPHON  Y 


them,  rendering  it  unnecessary  to  have  any  person  switch  toll  lines  at  the 
other  places. 

For  example,  A,  B,  C,  D,  and  E  may  be  five  exchanges  on  one  line  of 
railroad.  At  C  may  be  located  the  toll  board.  An  operator  there  can 
reach  out  over  the  toll  line,  dial  up  any  desired  connection,  and  control 
it.  Any  subscriber  in  a  small  town  A  can  automatically  call  the  toll 
operator  at  C  and  have  her  set  up  any  desired  connection,  either  to  a 
subscriber  in  C  to  one  in  D  or  E,  or  to  one  in  B. 


String  of  exchanges. 


It  is  better  if  traffic  is  large  enough  to  have  two  toll  lines  along  the 
same  route.  One  can  be  restricted  to  through  service  and  can  not  be 
called  by  the  subscribers,  it  is  under  the  control  of  the  operator  at  C. 
The  other  is  the  local  line  and  can  be  called  by  any  subscriber  at  the 
exchanges  along  the  line. 

One  plan  of  operation  gives  each  community  automatic  exchange  a 
selector  switch  whose  jacks  are  connected  to  the  toll  line  (local).  At 
exchange  A  level  one  is  connected  to  the  local  switches.     At  exchange  B 


T™ector°m  AUTOMATIC  EXCHANGE -"A  " 
Banks^.. 


AUTOMATIC  EXCHANGE-  8 


Local 
Conn 


Trunks  from 
Selector  tanks 


■  Toll  Conn 


\i\  \URepeaters 


Toll--    Y 
Selectors 


Line  Circuit -I 


Line  Circuit- 2 


Fig.  268. — Two  community  automatic  exchanges  with  free  automatic  trunks. 

level  2  is  connected  to  the  local  switches.  At  C  it  is  level  3,  at  D  level  4, 
and  at  E  level  5,  which  is  connected  to  the  local  switches. 

All  the  selectors  on  the  toll  line  are  operated  simultaneously  by  the  toll 
operator  in  calling  a  small  exchange.  All  lift  their  wipers  together  and 
stop  at  the  same  levels.  By  the  different  connections  described  above, 
they  select  and  connect  to  the  exchange  desired  and  to  no  other. 

Subscribers  at  any  small  exchange  call  the  toll  operator  at  C  by  loop 
dialing.  The  operator  dials  back  over  the  line  by  simplex  to  get  any 
exchange. 


RURAL  AUTOMATIC  TELEPHONES 


315 


Consider  the  case  of  two  adjacent  exchanges  which  have  free  service 
from  one  to  the  other.  (Fig.  268.)  Let  each  exchange  be  represented 
by  four  100-line  boards,  having  selectors  and  connectors. 

The  lines,  1  and  2,  which  carry  the  inter-exchange  traffic,  are  equipped 
at  each  end  with  a  selector,  and  are  in  addition  multipled  to  the  banks 
of  local  selectors.  A  repeater  is  inserted  between  the  selector  banks  and 
the  lines  for  the  purpose  of  holding  the  local  switches  from  releasing, 
feeding  common  battery  for  talking,  etc. 

The  banks  of  the  toll  selectors  are  connected  to  the  jacks  of  toll 
connectors,  one  or  more  in  each  100-line  board.  These  toll  connectors 
have  their  banks  multipled  to  the  banks  of  the  local  connectors,  so  that 
all  subscriber  lines  can  be  reached. 

When  a  subscriber  in  exchange  A  desires  to  call  one  in  B,  he  dials  a 
figure  which  causes  his  first  selector  to  seize  one  of  the  lines,  making  it 
busy  on  the  banks  of  all  the  other  first  selectors  in  both  exchanges  A  and 


Fig.  269. — Radial  system  of  exchanges. 

B.  He  then  dials  the  call  number  of  the  desired  telephone,  which  operates 
the  toll  selector  at  B  and  the  toll  connector  in  the  100-line  board  chosen. 

Radial  or  Satellite  Exchange  System. — The  radial  or  satellite  system 
of  exchanges  consists  of  a  central  exchange  connected  by  radial  trunks 
or  toll  lines  to  outlying  smaller  automatic  exchanges.  (Fig.-  269.)  The 
central  exchange  (manual  or  automatic)  is  the  controller  of  the  system. 
Although  each  community  automatic  exchange  completes  its  own  con- 
nections, it  depends  on  the  controlling  exchange  for  all  connections  to  the 
center  or  to  other  exchanges. 

Many  small  exchanges  are  still  operated  manually,  because  it  is 
necessary  to  keep  someone  there  to'make  toll  connections,  and  that  same 
person  can  answer  local  calls  in  addition  to  the  light  toll  business.  Cen- 
tralized toll  switching  removes  this  necessity  and  permits  the  small 
exchanges  to  be  made  automatic.  The  small  exchange  then  runs  much 
better  and  more  cheaply  than  with  manual  attendance. 


316 


AUTOMATIC  TELEPHONY 


Service  between  the  controlling  exchange  and  a  community  automatic 
exchange  may  be  free  or  a  charge  may  be  made.  If  the  service  is  free, 
the  two-way  trunks  from  the  community  exchange  will  run  directly  to 
the  local  board  in  the  controlling  or  " main"  exchange.  If  the  local  board 
is  manual,  the  trunk  may  terminate  as  a  subscriber  line  for  incoming 
traffic.  One  operator  may  handle  all  outgoing  calls  to  the  rural  exchange, 
she  alone  being  provided  with  a  calling  device.  It  is  possible  to  give 
each  "A"  operator  a  calling  device  so  that  she  may  dial  directly.  If 
the  local  board  is  automatic,  the  two-way  trunks  may  end  on  selectors 
for  incoming  traffic  and  selector  banks  for  outgoing  traffic. 

If  the  service  between  exchanges  requires  a  toll  charge,  the  radial 
trunks  will  terminate  as  toll  lines  on  the  toll  board  at  the  controlling 
exchange  and  in  a  toll  switch  in  the  community  automatic  exchange. 
This  rural  exchange  will  operate  a  ring-down  line  signal  on  the  toll  board 


Fig.  270. — Network  system  of  exchanges. 


and  the  toll  operator  will  record  the  call  in  the  standard  way,  after  which 
the  subscriber  will  release.  When  the  connection  is  ready  the  toll  operator 
will  call  back  through  the  toll  switches  and  thus  complete  the  line.  If 
certain  rural  lines  are  permitted  to  have  free  service,  a  special  tone  is 
provided,  so  that  the  toll  operator  can  discriminate  in  their  favor.  If 
the  local  conditions  warrant  it,  separate  trunks  can  be  provided  from  the 
community  automatic  exchange  running  directly  to  the  local  board  of 
the  controlling  exchange.  In  this  case,  the  rural  board  will  be  arranged 
so  that  only  certain  lines  can  gain  access  to  these  free  direct  trunks. 

Network  of  Exchanges. — The  network  system  of  exchanges  and  lines 
differs  from  the  above  in  that  there  are  direct  trunks  connecting  the 
community  automatic  exchanges  to  each  other  and  the  controlling  ex- 
change part  or  all  of  its  power  as  a  checking  agent.  (Fig.  270.)  Ad- 
jacent exchanges  may  call  each  other  freely.  These  trunks  are  two- 
way,  terminating  at  each  end  in  a  switch  and  the  banks  of  switches. 


RURAL  AUTOMATIC  TELEPHONES 


317 


Tandem  trunking  may  be  permitted  or  not,  as  desired.  If  permitted,  a 
subscriber  can  dial  a  call  from  his  own  exchange  A  through  one  or  more 
intermediate  exchanges  B  to  the  desired  exchange  C.  If  the  conditions  of 
business  require  a  toll  service  between  exchanges  so  far  apart,  the  incom- 
ing switches  (in  B)  will  not  be  equipped  with  outgoing  trunk  multiples. 
Thus,  while  local  lines  (in  B)  can  call  exchange  C,  a  call  from  A  into  B 
can  not  get  the  out  trunk  to  C  and  tandem  trunking  is  impossible.  The 
subscriber  in  A  must  call  the  controlling  exchange,  whose  toll  operator 
can  make  the  record  and  complete  the  connection. 

Further  details  in  the  application  of  automatic  switching  to  the  con- 
necting links  of  a  network  may  be  shown  by  considering  five  small  auto- 


Exchange-AH.) 
-Jo  Local  Switches 
I-A        I-M        I- 


R    Exchanqe-C(2>) 

— E — 


n—  Jo  Local  Switches 
I-A        I-D       I-M 


Fig.  271. — Typical  network  of  community  automatic  exchanges. 

matic  exchanges,  each  limited  to  1000  lines.  The  principles  shown  can 
be  applied  to  exchanges  of  any  size. 

Four  of  these  automatic  exchanges  (Fig.  271),  A,  B,  C,  and  D,  are 
supposed  to  be  located  approximately  at  the  corners  of  a  square  or 
rectangle.  Exchange  M  is  near  the  center  and  is  the  controlling  ex- 
change. Assume  further  that  free  service  is  given  between  all  adjacent 
exchanges,  but  that  a  charge  is  made  for  traffic  between  opposite 
corners  (A  to  D  and  B  to  C).  The  exchanges  are  numbered  from  1  to 
5  as  indicated. 

The  exchanges  are  connected  by  lines.  To  keep  the  illustration 
simple,  let  there  be  only  one  line  between  adjacent  exchanges — eight 
lines  in  all.  Any  such  link  can  easily  be  increased  by  adding  lines  and 
switches  without  changing  the  principles  involved. 


318  AUTOMATIC  TELEPHONY 

Each  exchange  has  two  kinds  of  first  selectors,  local  and  incoming. 
The  local  first  selectors  (illustrated  by  only  one  of  them,  L)  handle  all 
the  traffic  originated  by  the  subscribers  in  that  exchange.  One  bank 
level  carries  the  traffic  into  the  local  switches  to  local  subscribers.  Since 
this  is  a  small  system,  and  to  keep  the  numbering  uniform,  each  exchange 
has  a  different  level.  In  the  A  exchange  it  is  the  first  level,  in  B  it  is  the 
second  level,  etc.  The  other  levels  beside  the  local  level  lead  to  adjacent 
exchanges,  each  according  to  the  number  of  the  exchange.  From  the 
second  level  a  trunk  goes  through  a  repeater  R  to  the  line  leading  to  B, 
where  it  terminates  on  the  jacks  of  an  imcoming  selector  (I-A  from  A, 
I-D  from  D,  etc.). 

From  the  third  level  a  trunk  goes  through  repeater  R  to  the  line 
leading  to  exchange  C,  where  it  terminates  on  the  jacks  of  an  incoming 
selector  (I-A  from  A,  I-M  from  M,  etc.). 

The  repeater  inserted  in  the  outgoing  trunk  holds  the  local  switches 
in  the  originating  exchanges. 

The  incoming  selectors  (I-A,  I-B,  etc.)  have  the  correct  level  multi- 
pled  to  the  banks  of  the  local  selectors  L  so  that  they  have  access  through 
the  local  switches  to  the  local  subscribers. 

These  free  lines  are  really  two-way  trunks.  Each  end  of  a  line  is 
attached  to  an  incoming  selector  for  incoming  traffic  and  to  the  banks  of 
all  the  local  selectors  L  for  outgoing  traffic. 

Suppose  that  a  subscriber  in  exchange  A  calls  for  a  subscriber  in 
exchange  C,  whose  local  number  is  3225.  The  subscriber  in  A  will  first 
dial  the  figure  3  which  causes  his  first  selector  L  to  lift  its  wipers  to  the 
third  level  and  seize  the  line  to  C,  making  it  busy  at  both  ends.  Then  he 
dials  the  figure  3  of  the  local  number,  which  causes  the  wipers  of  the 
incoming  selector  I-A  to  rise  to  the  third  level  and  to  seize  an  idle  trunk 
to  second  selectors.  The  digit  2  chooses  the  hundred,  and  the  digits  2 
and  5  operate  the  connector  to  get  the  called  line. 

A  subscriber  in  exchange  C  who  calls  the  same  number  will  dial  the 
local  4-figure  number  only.     He  will  not  need  to  prefix  the  figure  3. 

To  handle  pay  traffic  between  A  and  D  and  between  B  and  C,  as 
well  as  between  the  network  and  the  long-distance  line  to  other  parts  of 
the  country,  a  toll  board  is  installed  at  exchange  M.  All  incoming  selec- 
tors in  M  have  the  fifth  level  multipled  into  the  banks  of  the  local  selectors 
L  because  all  the  other  exchanges  are  to  call  M  free* of  charge.  But  the 
tenth  or  "0"  level  is  run  separately  to  the  toll  board.  Any  call  for 
"Long  Distance"  made  by  a  C  subscriber  will  go  directly  to  the  toll 
board,  because  the  directory  lists  the  long-distance  operator  so  as  to 
secure  that  result.  It  may  be  listed  "50"  or  "00"  because  the  banks  of 
the  local  selectors  can  be  multipled  that  way  in  each  exchange. 

Trunks  from  incoming  selectors  at  M  are  kept  separate,  so  that  the 
toll  operator  knows  the  origin  of  each  call  that  comes  to  her. 


RURAL  AUTOMATIC  TELEPHONES  319 

If  a  subscriber  in  A  desires  to  call  someone  in  D,  he  is  instructed  by 
the  directory  to  call  "50,"  which  leads  his  call  to  the  toll  board.  The 
operator  makes  the  charges  and  completes  the  call.  The  operator  has 
lines  (not  shown  here)  leading  into  the  local  switches  through  which 
any  exchange  is  available.  If  desired,  toll  selectors  may  be  set  aside  for 
her  use.  If  the  identity  of  the  calling  station  is  in  question,  the  operator 
directs  the  calling  subscriber  to  hang  up  his  receiver.  Then  she  calls 
the  number  which  he  gave — this  establishes  the  identity.  The  knowledge 
that  this  check  may  be  made  at  any  time  deters  fraud. 

If  the  subscriber  at  A  tries  to  avoid  payment  by  calling  through  B  or 
C  to  reach  D  he  will  fail.  The  incoming  selector  I-A  in  both  these 
exchanges  does  not  have  any  connection  on  its  bank  for  the  D  exchange. 
It  is  impossible  to  call  in  that  way  out  of  that  place.  If  he  tries  to  dial 
from  A  through  M  to  D,  he  will  run  against  the  same  difficulty. 

It  is  possible  to  multiple  the  banks  of  local  selectors  so  that  if  a  sub- 
scriber tries  to  avoid  payment,  his  call  will  be  diverted  to  the  toll  board. 
This  is  done  by  connecting  the  level  which  will  be  called  to  the  level 
carrying  the  trunk  to  M.     (See  exchange  C.) 

Suppose  another  but  very  innocent  case.  A  subscriber  living  in  D 
has  much  business  with  a  subscriber  in  B,  whom  he  frequently  calls  free 
of  charge  by  dialing  "22"  and  the  rest  of  the  number.  If  that  subscriber 
in  D  moves  to  live  in  C  and  by  force  of  habit  dials  the  same  number  (22) 
as  before,  he  will  find  himself  answered  by  the  toll  operator  at  M,  who  will 
make  the  charge  and  complete  the  call.  Level  "2"  of  local  selectors  in 
C  is  multipled  to  level  5  and  the  trunk  to  M.  The  incoming  selector  I-C 
inilf  has  its  second  level  multipled  to  the  tenth,  and  thus  to  the  toll  board. 

Community  Automatic  Exchange. — In  the  above  descriptions  of  small 
rural  exchanges,  mention  was  made  of  the  community  automatic  ex- 
change.    An  account  of  its  characteristics  is  now  in  place. 

The  term  "  community  automatic  exchange, "  abbreviated  "C.A.X." 
is  now  applied  to  a  small  unattended  plant,  whose  toll  switching  and 
miscellaneous  services  are  handled  from  a  distance  and  whose  apparatus  is 
usually  designed  with  the  requirements  of  rural  lines  primarily  in  view. 
The  community  which  it  serves  may  be  the  farmers  of  a  region,  a  small 
town  or  village,  or  the  small  suburb  of  a  town.  The  traffic  is  chiefly 
local,  though  by  no  means  confined  to  local. 

The  rotary  lineswitch  is  standard  for  these  exchanges.  It  is  simple 
in  structure,  has  individual  control,  moves  very  little  in  its  work,  and 
has  access  to  a  larger  trunk  group  than  the  plunger  lineswitch. 

The  selectors  and  connectors  have  the  usual  structure,  and  in  general 
their  circuits  are  the  same  as  those  used  in  large  exchanges,  modified  to 
suit  the  operating  conditions  which  are  discussed  below. 

The  equipment  is  made  up  in  three  general  units,  the  switchboard, 
the  storage  battery,  and  the  charging  machine.     The  switchboard  (Fig. 


320 


A  UTOMA  TIC  TELEPHON  Y 


272)  is  made  in  two  or  three  parts,  which  are  installed  side  by  side  so  as  to 
form  one  unit.  These  parts  are  the  power-board  and  main  distributing 
frame,  the  local  board,  and  the  trunking  board.  The  last  is  omitted  if 
the  exchange  is  isolated  or  if  it  has  less  than  100  lines  and  the  connectors 
do  the  trunking.  This  is  the  case  in  the  illustration.  All  but  the  trunk 
board  have  been  fairly  well  standardized.  Because  of  the  many  variable 
conditions  found  in  different  places,  the  trunk  board  with  its  selectors 
and  repeaters  is  made  up  in  accordance  with  the  needs. 

In  Fig.  272  the  MDF  is  at  the  extreme  left,  with  the  power  board 
beside  it.     At  the  bottom  of  the  power  panel  is  the  solenoid  knife  blade 


Fig.  272. — Community  automatic  exchange  100-line  (front). 

switch  for  starting  the  charging  machine.  Above  it  is  the  ringing  ma- 
chine which  runs  by  battery  current,  next  the  counter-cell  switch,  and 
above  it  the  self-closing  reverse-current  circuit  breaker.  At  the  right 
of  the  power  board  are  the  rotary  lineswitches,  ten  per  shelf,  with  a 
terminal  assembly  at  the  top  of  the  frame  with  a  battery  panel  carrying 
fuses,  lamps,  etc. 

The  back  of  the  board  (Fig.  273)  shows  the  connectors  on  the  line- 
switch  frame  and  the  cabling  from  the  MDF  to  the  terminal  assembly. 
Near  the  center  of  the  power  board  is  the  small  motor  which  operates 
the  counter-cell  switch.  Almost  in  the  lower  left-hand  corner  of  the 
power  board  is  the  box  containing  the  voltage  governing  relays. 


RURAL  AUTOMATIC  TELEPHONES 


321 


The  charging  of  the  battery  is  done  preferably  by  a  motor-generator 
run  by  commercial  power.  The  battery  bus  bars  are  equipped  with  a 
pair  of  voltage  relays,  which  start  the  motor-generator  whenever  the 
pressure  falls  to  46  volts  and  which  stop  it  when  it  reaches  52  volts.  A 
self-closing  reverse  current  circuit  breaker  stands  between  the  generator 
and  the  battery.  When  the  generator  voltage  is  high  enough  to  be  safe, 
the  circuit  breaker  closes  the  circuit  and  the  charging  begins.  When  the 
motor  has  been  cut  off  by  the  high  voltage,  the  generator  voltage  dies 
away  until  a  slight  reverse  current  operates  the  circuit  breaker  and  opens 
the  circuit. 


Fig.  273. — Community  automatic  exchange  100-line  (back). 

The  power  board  circuits  are  shown  in  Fig.  274.  The  counter-cell 
switch  consists  of  a  heavy  copper  arm  mounted  on  a  shaft  which  rotates 
so  that  one  end  of  the  arm  passes  over  a  semicircular  row  of  contacts. 
These  are  of  heavy  construction  and  carry  wires  leading  to  the  counter 
cells.  The  arm  is  connected  to  the  negative  bus  bar  by  its  lower  end 
which  rests  on  a  semicircular  contact  bar. 

Inside  the  row  of  contacts  for  the  counter  cells  is  another  row  of 
smaller  contacts.  These  control  the  automatic  charging  switch.  When 
the  upper  end  of  the  counter-cell  switch  arm  rests  on  one  of  the  heavy 
contacts,  a  small  spring  brush  rests  on  the  small  contact  in  the  inner  row. 

A   small   series   motor   on   the    back   of   the   power   board   rotates 

21 


322 


AUTOMATIC  TELEPHONY 


the  counter-cell  switch  by  means  of  a  worm  gear.  The  motor  has  two 
opposing  fields  so  that  it  may  be  driven  in  either  direction. 

At  the  point  of  time  shown  in  the  illustration,  the  battery  is  being 
discharged  and  it  has  reached  the  point  where  four  counter  cells  are  still 
in  circuit.  The  two  knife  switches  which  control  the  charging  are 
supposed  to  be  closed.  The  voltage  relays  marked  "high"  and  "low" 
in  the  rectangle  in  the  lower  left-hand  corner  are  in  the  condition  shown. 
The  high  relay  is  de-energized,  but  near  the  operating  point.  The  low 
relay  is  energized,  but  almost  ready  to  fall  back. 

If  the  bus  bar  pressure  falls  to  46  volts,  the  low  relay  falls  back, 
pulling  up  relay  A  which  locks  itself,  pulls  up  the  low  relay  again,  and 
starts  the  switch  motor  to  rotating  the  counter-cell  switch  to  remove 
another  cell.  This  brings  back  the  bus  bar  voltage  to  a  point  above  46 
volts.  The  wheel  marked  D  with  its  springs  and  relay  C  form  the  center- 
ing device,  to  insure  the  accurate  centering  of  the  switch  arm  on  the 

Reverse  Current ' 
Circuit  Breaker 


Fig.   274. — Automatic  voltage  regulation  for  C.A.X. 

contacts.  When  the  shaft  rotates  a  little,  springs  S  close  and  energize 
relay  C.  When  the  switch  reaches  the  next  contact,  the  springs  S  open 
again,  cutting  off  battery  from  relays  C  and  A.  The  latter  falls  back  at 
once;  the  former  being  slow-releasing,  hangs  on  long  enough  to  insure 
that  A  has  released  before  restoring  the  battery  connection  for  A.  Relay 
A  stops  the  switch  motor. 

If  the  three  inside  contacts  on  the  counter-cell  switch  are  wired  to- 
gether as  shown,  the  cutting  out  of  the  cell  just  described  closes  the  circuit 
which  sends  battery  current  through  the  right  hand-coil  of  the  automatic 
charging  switch.  This  closes  the  commercial  power  leads  to  the  motor 
generator  and  the  charging  begins.  The  charging  switch  at  the  same 
time  opens  the  lead  which  operated  it  and  prepares  a  circuit  to  the  left- 
hand  coil. 

As  the  charging  proceeds,  the  voltage  of  the  battery  rises.  When  the 
bus  bar  voltage  reaches  52,  the  "high"  relay  pulls  up  and  operates 
relay  B.     The  latter  locks  itself,  cuts  off  relay  "high"  and  starts  the 


RURAL  AUTOMATIC  TELEPHONES  323 

switch  motor  to  cut  in  another  counter  cell.  The  centering  device 
operates  as  before  to  stop  the  switch  and  restore  the  relays  to  normal. 

When  the  charging  has  proceeded  to  the  point  at  which  all  the  counter 
cells  are  cut  into  service,  the  arm  of  the  switch  closes  the  circuit  to  the 
left-hand  coil  of  the  automatic  charging  switch.  The  latter  now  opens 
the  commercial  power  leads  and  stops  the  charging. 

In  regions  where  commercial  power  is  not  available  continuously,  the 
counter-cell  switch  is  arranged  to  start  the  charging  when  there  are  still 
two  or  three  cells  in  service.  It  is  done  by  connecting  to  the  starting 
wire  the  several  inside  contacts  as  indicated.  In  this  way  the  charge  is 
begun  some  time  before  the  battery  is  fully  discharged.  If  this  happens 
at  a  time  when  the  commercial  power  is  off,  there  is  reserve  enough  in  the 
battery  to  carry  the  switchboard  until  the  power  comes  on,  when  the 
motor  generator  will  start  up. 

If  for  some  reason  charging  does  not  begin  when  it  should  and  all 
counter  cells  have  been  cut  out,  the  lowest  inside  contact  will  close  the 
circuit  marked  "low  voltage  alarm"  which  notifies  the  people  in  the 
controlling  exchange  that  attention  is  needed. 

If  no  commercial  power  is  available,  a  gasoline  unit  is  installed. 
It  is  started  by  hand  as  often  as  necessary.  The  regular  maintainer 
need  not  do  it,  someone  who  lives  near  the  exchange  and  is  familiar  with 
gasoline  engines  is  employed  to  set  it  to  going.  The  stopping  is  auto- 
matic. The  preferred  method  is  to  put  in  the  engine  tank  a  measured 
amount  of  fuel,  according  to  a  chart  furnished.  The  amount  of  fuel  has 
been  proportioned  to  the  specific  gravity  of  the  battery.  When  the  fuel 
runs  out,  the  engine  stops  and  the  circuit  breaker  takes  care  of  the  cutting 
off  of  the  leads  to  the  battery. 

Another  method  is  sometimes  used.  The  fuel  tank  is  kept  full,  and 
the  engine  stopped  by  the  voltage  device,  when  all  counter  cells  have  been 
cut  into  circuit. 

In  case  the  charging  must  be  more  or  less  irregular,  larger  capacity 
storage  battery  is  used. 

Regardless  of  the  kind  of  charging  used,  the  battery  is  given  its 
regular  overcharge  as  specified  by  the  manufacturer  of  the  battery. 
This  is  necessary  to  insure  the  health  of  the  plates  and  electrolyte. 

The  design  of  apparatus  for  a  community  automatic  exchange  is  very 
largely  determined  by  the  following  factors: 

1.  Transmission Local  battery  or  common  battery. 

2.  Line  relay Double-wound  or  ground  relay. 

3.  Line  equipment Lineswitch  or  selector  (or  connector). 

4.  Signalling Selective  or  code. 

5.  Signal  control Push-button  or  periodic. 

6.  Reverting  calls Push-button,  periodic,  or  hand  generator. 

7.  Battery  charging  means Motor  generator  or  gasoline  engine. 

8.  Battery  charging  method Float  or  hand  start  with  automatic  stop. 


324  AUTOMATIC  TELEPHONY 

The  tendency  is  to  use  common  battery  transmission  as  far  as  possible. 
The  replacement  of  dry  cells  is  a  great  and  increasing  expense.  It  is  to 
be  incurred  only  if  the  salvage  of  old  equipment  causes  it  to  seem  to  be 
more  profitable  to  retain  the  local  battery,  or  if  the  lines  are  so  long  that 
it  is  necessary  for  good  transmission.  As  far  as  automatic  operation  is 
concerned  it  makes  no  difference  which  is  used.  Most  lines  in  rural 
operation  are  long  enough  to  require  a  special  weak  current  common 
battery  transmitter.  In  all  cases  the  polarized  receiver  is  necessary, 
because  the  subscriber  must  be  able  to  listen  through  a  condenser  to  see 
if  the  line  is  in  use. 

The  line  relay  which  handles  the  impulses  should  be  connected  to  the 
grounded  terminal  of  the  battery  as  described  above.  If  the  line  condi- 
tions are  as  good  as  in  larger  exchanges,  the  usual  double-wound  line 
relay  is  fully  adequate. 

The  usual  practice  is  to  carry  all  lines  in  the  community  automatic 
exchange  to  lineswitches.  All  selectors  and  connectors  are  usually 
equipped  with  the  ground  relay  circuit,  so  as  to  render  repeaters 
unnecessary. 

In  regions  where  the  rural  subscribers  have  been  using  local  battery 
magneto  telephones  for  years  and  are  accustomed  to  code  signals  the 
code  signal  can  be  used  with  the  C.A.X.  The  non-secret  nature  of  the 
call  is  regarded  by  many  subscribers  as  a  positive  advantage.  In  rural 
regions  there  is  a  community  of  interest  and  a  mutual  helpfulness 
which  is  difficult  for  the  city  dweller  to  realize.  More  congested  rural 
regions  seem  to  require  greater  secrecy,  and  the  selective  signal  is  favored 
and  sometimes  even  requested. 

Push-button  ringing  and  periodic  ringing  by  the  central  office  apparatus 
are  the  same  as  described  above. 

Reverting  calls  are  handled  in  accordance  with  the  method  of  signal- 
ling used.  If  local  battery  magneto  telephones  with  simplex  dialing 
are  employed,  the  subscribers  ring  each  other  just  as  they  did  on  the 
manual  switchboard.  If  the  signalling  is  selective,  the  subscriber  dials 
a  special  number  and  receives  alternate  signals  as  described  above.  If 
code  signalling  is  used,  it  maj^  be  either  periodic  or  push-button  controlled. 
The  latter  requires  reverting  selectors  available  to  the  party  lines. 


CHAPTER  XIV 

CUT-OVERS  AND  INTERCONNECTING  OF  MANUAL 
AND   AUTOMATIC   OFFICES 

Cutover  from  a  One-office  Common  Battery  Manual  Plant  to  an 
Automatic  Plant. — The  arrangements  required  for  giving  service  during 
the  process  of  cutting  subscribers  from  a  single,  common  battery,  manual 
central  office  to  a  single  automatic  central  office  are  very  simple. 
Since  all  automatic  telephone  instruments  are  essentially  the  same  as 
common  battery  manual  instruments,  with  the  exception  of  the  auto- 
matic calling  device  which  is  connected  in  the  circuit  at  the  time  of  call- 
ing only,  one  of  these  instruments  may  at  any  time  be  substituted  for  the 
regular  manual  instrument  on  a  line  to  a  common  battery  manual  board. 
Therefore,  subscribers,  stations  using  wall  or  desk  telephones  only  are 
prepared  for  a  cut-over  by  simply  taking  out  the  manual  instruments 
and  replacing  them  with  the  automatic  instruments,  one  at  a  time. 
Until  the  cut-over  takes  place,  each  subscriber  uses  his  automatic  instru- 
ment just  as  he  had  hitherto  used  his  manual  instrument. 

This  being  the  case  the  automatic  central  office  equipment  may  be 
installed  in  the  building  provided  for  it,  and  the  subscribers'  cables 
multipled  into  the  main  distributing  frame  of  the  automatic  office, 
where  each  circuit  should  be  kept  open,  until  the  hour  for  the  cut-over 
arrives,  at  the  protector  springs  on  the  main  distributing  frame  or  at  the 
bridge  cut-off  relay  springs. 

This  may  be  easily  done  by  inserting  small  insulators,  such  as  wooden 
tooth-picks,  between  the  springs.     (See  Fig.  275.) 

Pending  the  cut-over  each  subscribers'  line  should  be  temporarily 
switched  from  the  manual  to  the  automatic  switchboard  by  inserting 
insulators  at  the  main  frame  in  the  manual  central  office,  and  removing 
the  insulators  at  the  automatic  central  office,  and  the  operation  of 
the  telephone  should  be  tested  by  having  an  employee  operate  the  sub- 
scribers' station  equipment  while  another  supervises  the  central  office 
apparatus.  After  this  test  has  been  made,  the  line  should  be  cut  back 
to  the  manual  switchboard  to  operate  manually  until  the  hour  for  the 
cut-over  is  reached. 

It  is  customary  to  notify  the  subscribers  through  the  daily  papers  and 
by  special  notices  that  the  cut-over  will  take  place  at  a  certain  hour. 
Preparatory  to  this  time  each  subscriber  must  be  supplied  with  a  direc- 
tory, showing  what  the  various  subscribers'  numbers  will  be  when  auto- 

325 


326 


A  UTOMA  TIC  TELEPHONY 


matic  service  is  inaugurated,  and  be  instructed  in  the  use  of  the  dial, 
so  that  after  the  hour  on  which  he  is  informed  that  the  cut-over  is  to 
take  place  he  will  understand  that  he  is  to  use  the  calling  device  for  secur- 
ing his  connections  and  will  be  in  possession  of  the  information  which  will 
enable  him  to  do  so  properly. 

At  the  central  office  the  process  of  cutting  over  consists  in  removing 
the  insulators  from  the  protector  bridge  cut-off  relay  springs,  and  in 


Fig.  275. — A  lineswitch  unit  (100  lines)  with  toothpicks  in  the  line  and  cut-off  relays, 
tied  together  ready  to  be  pulled  out  at  the  cut-over  signal. 

cutting  the  lines  of  inserting  similar  insulators  in  the  manual  central 
office.  The  circuit  of  the  individual  master  switches  are  temporarily 
opened,  the  plungers  being  aligned  opposite  the  first  trunk.  Should  any 
permanents  develop,  when  the  cut  is  made,  the  associated  lineswitches 
will  plunge  in  on  comparatively  few  first  selectors,  after  which  the  master 
switch  circuits  are  closed  and  normal  traffic  flows  to  the  remaining 
selectors. 


CUT-OVERS  AND  INTERCONNECTING  OFFICES  327 

Sometimes  subscribers'  lines,  which  have  boon  served  from  one 
manual  central  office,  are  distributed  among  several  offices,  for  example, 
a  main  central  office  and  one  or  more  district  stations  when  the  cut- 
over  to  automatic  equipment  is  made. 

A  quick  and  satisfactory  cut-over  may  be  made  under  these  condi- 
tions, however,  just  as  when  the  change  is  made  to  one  automatic  central 
office  only.  If  fact,  the  course  of  procedure  is  the  same,  with  the  excep- 
tion that  each  subscriber'  line  must  be  multipled,  prior  to  the  cut-over, 
into  the  particular  automatic  office  to  which  it  is  to  be  eventually  con- 
nected permanently  and  these  be  temporarily  kept  open  until  the  hour 
for  cut-over  arrives. 

Furthermore,  the  trunks  for  automatic  calling  between  offices  must  be 
prepared  and  thoroughly  tested  out  prior  to  the  cut-over  along  with 
the  equipment  installed  in  each  office. 

Changing  Subscribers'  Station  Apparatus. — Subscribers'  stations 
equipped  with  private  branch  exchange  switchboards  should  be  prepared 
for  the  cut-over  by  remodeling  them  so  that  they  may  be  used  in  connec- 
tion with  trunks  to  either  a  manual  or  automatic  switchboard  up  to  the 
time  of  the  cut-over.  After  the  cut-over  it  is  generally  desirable  to  make 
further  changes  in  order  to  furnish  better  supervision  to  the  "P.B.X. " 
operators,  and  generally  to  simplify  the  operation  of  the  equipment.  It 
is  apparent  that  if  the  incoming  trunks  to  the  private  branch  exchange  are 
equipped  with  ring-up  signals  of  any  kind,  as  they  usually  are,  whether  the 
P.B.X.  has  been  used  in  connection  with  a  magneto  or  common  battery 
manual  system,  that  the  same  signal  may  be  operated  when  the  trunks 
are  connected  to  the  banks  of  the  automatic  connector  switches,  which  also 
signal  called  stations  by  the  use  of  alternating  ringing  current.  It  is 
necessary,  however,  to  see  that  a  condenser  is  installed  in  series  with 
each  ring-up  signal,  for  otherwise  the  ringing  relay  of  the  connector  switch 
which  connected  to  a  trunk  would  operate  as  soon  as  connection  was 
established,  and  calls  from  desk,  metered  lines  or  pay  stations  would 
operate  supervisory  or  registering  devices  prematurely,  i.e.,  previous  to 
the  actual  answering  of  the  call.  Also  in  two-way  two-wire  trunks  it  is 
necessary  that  the  loop  be  normally  open. 

For  calls  going  in  the  other  direction,  a  calling  device  must  be  supplied 
to  the  "  P.Z2. ^.''operator  and  be  connected  up  so  that  after  the  hour  for 
the  cut-over  has  arrived  she  can  call  parties  by  using  it.  In  almost  every 
instance  this  can  be  done  very  easily  by  connecting  the  calling  device 
to  the  ringing  bus  bars  of  the  calling  side  of  the  operator's  cord  circuits, 
and  by  inserting  a  master  key  which  will  enable  the  operator  to  switch 
either  the  calling  device  or  the  ringing  machine  onto  the  bus  bars.  It 
must,  of  course,  be  remembered  that  as  an  automatic  call  proceeds,  and 
after  its  completion,  it  is  necessary  to  keep  the  calling  subscriber's  loop 
closed  in  order  to  prevent  the  connection  from  releasing.     No  special 


328  AUTOMATIC  TELEPHONY 

provision  is  required  for  this,  however,  in  some  ordinary  common  battery 
manual  P.B.X.'&,  using  22-volt  batteries,  because  the  regular  cord  circuit 
relays  arranged  for  supplying  battery  to  a  calling  plug  furnish  the  bridge 
required.  Another  and  perhaps  a  better  way  to  arrange  one  of  these 
stations  for  a  cut-over,  where  conditions  will  permit,  is  to  install  trunk 
jacks  with  the  associated  supervisory  relays,  as  shown  in  Fig.  158  in  the 
chapter  on  subscribers'  station  equipment  for  automatic  exchanges. 

As  a  rule  the  equipment  of  this  trunk  will  serve  on  a  straight  manual 
trunk  and  the  cord  circuits  used  in  this  P.B.X.  are  similar  to  many  that 
are  used  in  straight  manual  systems.  As  a  result  of  many  years'  observa- 
tion it  may  be  stated  that  90  per  cent  of  the  trouble  following  a  cut-over 
to  automatic  will  develop  in  the  P.B.X.'s  and  Intercommunicating 
Systems.  For  this  reason  it  is  imperative  that  no  pains  be  spared  when 
testing  out  this  equipment. 

Changing  Service  Desks  and  Toll  Boards. — The  wire  chief's  desk 
complaint  desks,  and  information  clerks'  desks  are  generally  not  re- 
modeled when  a  cut-over  is  made  from  manual  to  automatic  equipment, 
but  as  a  rule  new  desks,  especially  designed  for  use  with  automatic 
apparatus,  are  installed  and  the  old  desks  are  abandoned. 

This  is  generally  the  practice  in  connection  with  long-distance  switch- 
boards also,  although  long-distance  switchboards  are  sometimes  re- 
modeled for  use  in  connection  with  the  new  automatic  equipment.  As 
a  rule,  when  the  manual  switchboard  in  connection  with  which  the  wire 
chief's,  information,  and  long-distance  switchboards  have  been  operated 
has  reached  a  condition  where  a  new  switchboard  is  required,  all  of  the 
boards  used  in  connection  with  it  are  ready  for  the  scrap  heap  also,  or  are 
so  outgrown  or  out  of  date  that  it  would  not  pay  to  remodel  them  for 
use  in  the  new  central  office.  When  it  is  decided  to  use  any  of  these 
boards,  it  requires  a  special  study  in  each  case  to  determine  the  best 
means  of  bridging  the  cut-over  period. 

Cutting  Over  from  a  Magneto  Manual  System  to  a  Common  Battery 
Automatic  System. — The  procedure  in  cutting  over  a  magneto  system  is 
generally  the  same  as  that  outlined  in  the  foregoing  portion  of  this 
chapter,  with  the  exception  that  at  each  subscribers'  station  equipped 
with  a  desk  or  wall  instrument  only,  it  is  preferable  to  install  a  new  auto- 
matic instrument  beside  the  old  magneto  instrument.  The  latter  should 
not  be  removed  until  after  the  cut-over,  because  it  is  necessary  to  have 
the  magneto  for  signalling,  and  the  local  battery  of  the  instrument  for 
furnishing  talking  current  until  after  the  cut-over  takes  place.  Prior 
to  the  cut-over  the  automatic  instrument  should  be  bridged  across  the 
line  and  the  ringer  of  the  magneto  instrument  should  be  disconnected 
so  that  all  calls  incoming  will  ring  the  bell  of  the  automatic  instrument, 
while  the  subscriber  should  be  instructed  to  respond  at  the  magneto 
instrument.     After  the  hour  for  the  cut-over  has  passed,  the  subscriber 


CUT-OVERS  AND  INTERCONNECTING  OFFICES  329 

simply  stops  using  the  magneto  telephone  and  uses  the  automatic  instru- 
ment for  all  of  his  calls.  The  telephone  company  removes  the  old 
magneto  instruments  as  rapidly  as  possible. 

Changing  a  Multi-office  Manual  System  to  Automatic  Equipment. — 
When  all  of  the  subscribers'  lines  connected  to  the  various  offices  in  a 
multi-office  manual  system  are  to  be  cut-over  to  new  automatic  switch- 
boards at  one  time,  the  problems  encountered  are  the  same  as  those 
mentioned  in  the  preceding  portion  of  this  chapter.  In  other  words,  the 
task  is  increased  in  magnitude,  but  not  in  complexity.  If.  however,  some 
of  the  offices  are  to  be  operated  manually  after  one  or  more  offices  have 
been  changed  to  automatic  equipment,  new  problems  arise. 

Taking  a  comparatively  simple  case,  for  example,  suppose  that  a 
given  city  contains  two  large  central  offices;  that  the  switchboard  in  one 
of  these  has  reached  the  end  of  its  life,  while  the  switchboard  in  the  other 
is  in  comparatively  good  condition  and  may  be  operated  for  some  years 
longer,  that  the  company  owning  the  system  has  not  the  financial  means 
to  change  both  offices  at  once  and  full  automatic  equipment,  and  that  it 
has  therefore  decided  to  immediately  install  automatic  apparatus  in 
place  of  the  worn-out  switchboard,  but  to  continue  to  operate  the  good 
manual  switchboard  for  some  years  longer.  So  far  as  switching  over 
the  lines  of  the  subscribers  connected  to  the  office  which  is  to  be 
abandoned  and  so  far  as  arranging  for  intercommunications  between  them 
are  concerned,  the  matter  may  be  handled  just  as  if  the  office  were  the 
only  one  in  the  system;  but  arrangement  must  be  made  for  handling 
calls  from  the  new  automatic  office  to  the  manual  office  which  is  to  be 
retained,  and  vice  versa,  from  the  manual  office  to  the  automatic  office. 
These  arrangements  must  be  such  that  it  will  not  be  difficult  for  the 
subscribers  to  either  office  to  understand  how  to  secure  subscribers  to  the 
other,  and  such  that  the  service  in  either  direction  will  be  rapid,  otherwise 
satisfactory  and  economical.  Under  these  conditions  the  calls  to  be 
trunked  in  either  direction  may  be,  and  have  been  in  practice,  handled  in 
any  one  of  several  different  ways. 

Plan  1.  Calls  Going  From  the  Automatic  to  the  Manual  Office. — 
One  of  the  simplest  methods  for  handling  the  calls  from  the  automatic  to 
the  manual  office,  and  one  which  has  been  used  with  success  in  a  number 
of  different  places,  is  to  arrange  the  trunks  so  that  in  the  automatic  office 
they  will  terminate  in  one  level  of  the  first-selector  banks,  enabling  any 
automatic  subscriber  to  secure  a  trunk  to  the  manual  office,  by  making 
one  turn  of  his  calling-device  dial;  while  in  the  manual  office,  they  will 
terminate  either  in  regular  subscribers'  line  jacks,  or  preferably,  in  cords 
and  plugs  before  "B"  operators.  If  these  trunks  terminate  in  regular 
line-jack  equipments  they  may  be  distributed  among  the  "A"  operators 
positions,  but  if  this  will  over-load  the  "A  "  operator,  the  positions  which 
were  formerly  used. as  "B"  positions,  before  the  old  manual  switchboard 


330  AUTOMATIC  TELEPHONY 

was  abandoned,  should  be  continued  as  "B"  positions,  and  the  trunks 
be  terminated  in  the  cords,  either  before  or  just  after  the  cut-over,  to 
provide  all  supervisory  features  desired.  For  example,  if  the  automatic 
switchboard  is  of  the  Automatic  Electric  Company's  type  and  uses 
reversing  battery  connectors,  then  either  the  " B"  operators  cord- 
circuits,  or  repeaters  used  on  the  trunks  from  the  automatic  office  to  the 
manual  office,  should  be  arranged  so  that  when  a  manual  party  responds 
to  a  call,  he  will  reverse  the  direction  of  current  flow  in  the  calling  sub- 
scriber's loop. 

The  foregoing  description  should  make  the  method  of  operation 
apparent,  when  it  is  said  that  each  subscriber  is  instructed  in  his  directory, 
and  also,  if  possible,  by  a  notice  printed  on  the  number  disk  of  his  dial 
device,  that  by  turning  his  dial  from  a  certain  finger  hole  (for  example, 
finger  hole  6)  he  will  secure  an  operator  in  the  manual  office. 

In  the  directory  the  word  "Automatic"  should  be  printed  in  front  of 
all  numbers  belonging  to  automatic  telephones,  and  the  word  "Operator," 
or  "Manual,"  or  the  name  of  the  manual  office  or  nothing  at  all,  may  be 
printed  in  front  of  the  numbers  belonging  to  manual  telephones.  It  is 
found  that  it  is  a  comparatively  easy  matter  to  teach  subscribers  that 
when  one  wishes  connection  to  any  number  beside  which  the  word  "Auto- 
matic" is  printed,  that  he  makes  the  connection  by  means  of  his  auto- 
matic calling  device  in  the  usual  way;  while,  if  he  wishes  connection  to  any 
number  connected  to  the  manual  switchboard,  he  turns  his  dial  from 
finger  hole  and  gives  the  number  desired  to  the  operator  who  responds, 
and  who  then  completes  the  connection  and  rings  the  desired  party  just 
as  in  the  regular  manual  practice. 

Plan  2. — A  plan  which  is  more  economical  of  operation,  because 
it  eliminates  the  "J3"  operator  at  the  manual  switchboard,  is  to  install 
in  the  manual  office  enough  automatic  selector  and  connector  switches 
with  the  banks  of  the  connector  switches  multipled  to  the  multiple  of 
the  manual  switchboard,  to  enable  the  automatic  subscribers  to  call  all 
manual  numbers  automatically.  This  plan  is  generally  warranted  where 
a  large  percentage,  333^  per  cent  or  more,  of  the  connections  completed  in 
the  manual  office  originate  in  the  automatic  office,  and  where  the  manual 
office  is  to  be  retained  for  more  than  two  or  three  years. 

An  arrangement  of  this  kind  has  been  worked  very  successfully  in 
several  large  American  offices  during  a  number  of  years,  pending  a 
change  of  the  manual  office  to  full  automatic.  With  this  plan  the  trunks 
from  the  automatic  to  the  manual  office  should  terminate  in  first  selector 
banks  in  the  automatic  office  as  in  Plan  1,  and  as  in  regular  multi-office 
automatic  practice;  while  in  the  manual  office  they  should  terminate  in 
second  selector  switches. 

Generally  a  system  of  the  character  under  discussion  would  be  of 
such  size  that  third  selectors  also  would  be  installed  in  each  office  and 


CUT-OVERS  AND  INTERCONNECTING  OFFICES  331 

five-figure  automatic  numbers  would  be  used.  To  make  this  system 
practicable  it  will  be  necessary,  of  course,  to  change  some  of  the  numbers 
in  the  manual  office,  because  the  numbering  of  manual  switchboard  lines 
generally  starts  with  No.  1 ;  whereas,  in  automatic  practice  each  number 
has  the  same  number  of  digits  as  each  other  number.  Consequently 
all  manual  numbers  having  less  than  four  digits,  that  is,  numbers  from  1 
to  999,  must  be  changed  to  at  least  four-digit  numbers.  If  the  system 
is  such  that  third  selectors  are  used,  requiring  five-digit  numbers,  either 
a  figure  or  a  letter  must  be  prefixed  to  all  of  the  four-figure  manual 
numbers.  As  a  rule  matters  are  simplified,  and  subscribers  are  mollified, 
by  using  a  letter  prefix  and  leaving  the  balance  of  the  number  unchanged. 
It  is  advisable  to  consider  using  the  first  letter  of  the  manual  office  name 
as  the  prefix.  With  this  arrangement  it  is  unnecessary  for  an  automatic 
subscriber  to  know  whether  the  party  he  wishes  to  call  is  connected  to 
the  automatic  switchboard  or  to  the  manual  switchboard.  He  makes 
the  call  and  secures  his  party  automatically  in  either  case. 

The  connector  banks  and  the  manual  multiple  jacks  are  connected 
together  in  such  a  way  that  if  the  line  is  made  busy  at  either  place,  it 
will  be  guarded  at  the  other.  As  a  typical  example  of  a  practical  inter- 
connection of  connector  switch  circuits  and  banks  with  a  multiple  manual 
switchboard  Fig.  276  shows  circuits  for  multipling  together  connector 
switches  of  the  Automatic  Electric  Company's  type  and  switchboard 
circuits  of  the  Western  Electric  Company's  No.l  type  board.  There  is 
no  change  in  the  circuits  of  the  Western  Electric  board,  and  the  only 
change  from  standard,  full  automatic  practice  in  the  circuits  of  the  con- 
nector switch  is  due  to  the  fact  that  in  the  Western  Electric  board,  as 
in  some  other  manual  switchboards,  a  busy  line  is  guarded  by  connecting 
the  sleeve  of  the  jack  to  negative  battery;  whereas,  in  regular  automatic 
practice  the  private  contact  of  the  guarded  line  is  always  connected  to 
earth,  that  is,  to  the  positive  terminal  of  the  battery.  It  therefore 
becomes  necessary  to  add  the  430-ohm  resistance  coil  X  to  the  connector 
switch  and  to  make  some  slight  alterations  in  the  circuit.  Since,  other- 
wise, it  is  practically  the  same  as  that  shown  and  described  in  Chapter  III, 
only  the  operation-  in  connection  with  the  manual  switchboard  will  be 
explained  here. 

It  will  be  noted  that  the  positive  side  of  the  connector-bank  multiple 
is  connected  directly  to  the  tip  side  of  the  manual  switchboard  multiple, 
the  negative  side  of  the  connector  bank  is  connected  to  the  ring  side  of 
the  manual-switchboard  multiple,  and  the  corresponding  private  contacts 
of  the  connector  bank  are  connected  to  the  sleeve  multiple  of  the  manual 
switchboard.  The  result  is  that  whenever  the  connector  switch 
completes  connection  to  any  line,  the  sleeves  of  all  the  corresponding 
jacks  and  the  private  bank  contacts  are  connected  to  negative  battery, 
through  the  private  wiper,  make-springs  relay  W  and  resistance  coil  X. 


332 


AUTOMATIC  TELEPHONY 


CUT-OVERS  AND  INTERCONNECTING  OFFICES  333 

On  the  other  hand,  whenever  a  connector  switch  finds  a  line  made  busy, 
in  one  of  its  jack,  in  the  manual  switchboard,  a  circuit  may  be  traced 
from  negative  battery  of  the  manual  switchboard,  through  the  lamp  and 
contact  of  cord  relay  B,  resistance  coil  2,  the  sleeve  of  the  plug,  the  sleeve 
of  the  jack,  the  private  bank  multiple,  private  wiper,  break-springs 
relay  W,  make-springs  relay  A  (relay  A  has  not  yet  de-energized,  follow- 
ing the  rotary  impulses),  break-springs  relay  W,  busy  relay  B  to  positive 
battery.  The  result  is  that  the  busy  relay  closes  a  circuit  from  the 
busy  bus  bar  to  the  positive  side  of  the  calling  subscriber's  loop,  at  the 
same  time  closing  a  locking  circuit  for  itself  which  may  be  traced  from 
ground,  relay  B,  break-springs  relay  W,  break-springs  relay  A  (which  by 
this  time  has  de-energized),  make-springs  relay  B,  resistance  X  to  nega- 
tive battery.  Thus  it  will  be  seen  that  the  calling  subscriber  will  con- 
tinue to  receive  the  busy  tone  even  though  the  called  line  should  become 
idle.  In  addition  to  the  foregoing  the  busy  relay  also  opens  the  circuit 
to  the  rotary  magnets,  in  order  that  there  can  be  no  further  rotation  of 
the  switch  should  the  subscriber  again  operate  his  calling  device. 

When  the  calling  subscriber  hears  the  busy  signal,  he  replaces  the 
receiver  on  the  switch-hook  and  all  connections  are  restored  to  normal 
condition,  as  usual. 

When  an  automatic  subscriber  causes  a  connector  switch  to  advance 
its  wipers  into  engagement  with  a  set  of  bank  contacts  associated  with 
an  idle  manual  line  a  circuit  may  be  traced  as  follows:  From  ground 
through  the  cut-off  relay  of  the  manual  switchboard,  private  multiple, 
private  wipers,  break-springs  relay  W,  break-springs  relay  A  (which  has 
de-energized  following  the  last  of  the  rotary  impulses),  through  the  low- 
resistance  winding  of  relay  W,  break-springs  relay  B,  through  resistance 
X  to  negative  battery.  Relay  W  will  energize  sufficiently  over  this  circuit 
to  close  the  locking  springs  X,  following  which  a  circuit  may  be  traced 
from  ground,  make-springs  relay  B,  make-springs  relay  W,  through  the 
high  resistance  of  relay  W,  to  negative  battery.  The  current  flowing  in 
this  circuit  will  cause  relay  W  to  become  fully  operated  and  accomplish 
the  following  results: 

(a)  Place  negative  battery  through  resistance  X  directly  on  the 
private  multiple,  in  order  to  busy  the  line. 

(6)  Cut  the  ringing  relay  C  through  to  the  wipers. 

(c)  Open  the  circuit  to  the  low  winding  of  relay  W. 

(d)  Open  the  circuit  to  the  busy  relay  B. 

(e)  Open  the  rotary  circuit  to  prevent  further  rotation  should  the 
calling  subscriber  again  operate  his  dial. 

The  remainder  of  the  connector  operation  is  standard,  and  need  not 
be  repeated  here. 

Calls  Going  from  the  Manual  Office  to  the  Automatic  Office. — Calls 
originating  in  the  manual  office  for  subscribers  in  the  automatic  office 


334  AUTOMATIC  TELEPHONY 

may  be  handled  in  cither  one  of  several  different  ways,  which  have  been 
found  by  experience  to  be  good  practice. 

Plan  1. — One  of  the  simplest  methods,  where  conditions  warrant  it, 
is  to  use  the  " B"  operators'  sections,  and  as  much  of  the  balance  of  the 
old  abandoned  manual  switchboard  as  may  be  necessary  to  make  a  " B" 
switchboard  to  be  installed  alongside  of  the  automatic  switchboard  for 
handling  these  calls.  To  carry  out  this  plan,  the  multiple  jacks  of  the 
switching  section  are  interconnected  with  the  connector  banks  of  the 
automatic  switchboard,  in  accordance  with  circuits  in  Fig.  276  just 
described,  or  according  to  any  similar  scheme.  The  old  order  wires 
from  the  "A  "  operator  in  the  retained  manual  office  to  the  "B"  positions 
of  the  abandoned  switchboard  are  used  for  ordering  up  connections, 
which  the  "  B"  operators  complete  by  plugging  into  the  multiple  jacks  in 
the  usual  manner.  It  is  apparent  that  one  difficulty  with  this  scheme  is 
to  make  such  arrangement  that  the  "Z?"  positions  may  be  used  in  hand- 
ling calls  between  the  two  manual  switchboards  up  to  the  moment  of  the 
cut-over,  and  may  be  available  for  use  as  a  switching  section  in  multiple 
with  the  automatic  switchboard  immediately  after  the  cut-over.  The 
problem  may  be  a  comparatively  easy  one,  however,  if  the  new  automatic 
switchboard  is  in  the  same  room,  or  in  the  same  building,  as  the  manual 
switchboard  which  is  to  be  abandoned.  The  details  of  the  plan  must,  of 
course,  depend  upon  the  conditions  peculiar  to  each  case. 

Plan  2. — A  second  plan  for  completing  the  calls  trunked  to  the  auto- 
matic central  office  is  to  use  a  " B"  operator's  switchboard  which  is  not 
equipped  with  any  multiple  jacks,  but  is  supplied  with  automatic  calling 
devices.  The  trunks  entering  the  "B"  positions  do  not  terminate  in 
cords  and  plugs,  but  pass  through  the  " B"  operator's  positions  and  termi- 
nate in  lineswitches,  or  first  selector  switches  in  the  automatic  switch- 
board. 

Each  trunk  is  equipped  in  the  position  of  the  " B"  operator,  through 
which  it  passes,  with  keys  necessary  for  switching  calling  devices  into 
connection  with  it.  This  plan  is  operated  by  the  "A"  operators  at  the 
manual  office,  ordering  up  connections  over  order  wires  just  as  in  Plan  1. 
The  " B"  operator  secured  assigns  a  trunk,  as  in  regular  manual  practice, 
and  instantly  throws  a  key  which  switches  one  of  her  calling  devices, 
which  is  idle,  onto  the  trunk,  and  proceeds  to  call  the  number  of  the 
desired  party. 

When  the  call  has  been  completed,  if  the  desired  line  should  be  busy, 
the  busy  signal  will  be  transmitted  automatically,  by  the  connector 
switch  used,  back  to  the  calling  subscriber.  If  the  called  party  should 
not  be  busy,  the  connector  switch  will  automatically  signal  him,  and  the 
"B"  operator  need  pay  no  further  attention  to  the  connection.  The 
supervisory  arrangements  should  be  such  that  when  the  "A"  operator 
plugs  into  the  jack  of  the  trunk  assigned,  a  lamp,  corresponding  to  the 


CUT-OVERS  AND  INTERCONNECTING  OFFICES  335 

trunk,  will  glow  in  the  "B"  operator's  position  and  the  regular  calling 
cord,  supervisory  lamp  should  light  at  the  "A"  operator's  position. 
When  the  "B"  operator  throws  her  calling-device  key,  the  signal  lamp 
should  go  out,  and  a  guard  lamp  associated  with  the  trunk  should  light 
and  remain  lit,  so  long  as  the  connection  is  up,  to  prevent  the  "B" 
operator  from  reassigning  the  trunk  while  it  is  engaged. 

When  the  called  party  responds,  the  calling-cord  lamp  in  the  "A" 
operator's  position  should  go  out.  Either  party  should  be  able  to  flash 
the  corresponding  cord  lamp  in  the  "A"  operator's  position  at  any  time, 
by  moving  his  receiver  switch-hook  up  and  down  slowly.  When  either 
party  hangs  up  the  receiver,  the  corresponding  disconnect  lamp  should 
glow  in  the  "A"  operator's  position,  and  when  she  pulls  down  the  con- 
nection the  guard  lamp,  associated  with  the  trunk  used  in  the  "B"  op- 
erator's position,  should  be  extinguished.  All  of  these  features  are 
easily  arranged. 

If  the  calling  device  used  by  the  "B"  operator  is  of  the  push-button 
type,  so  that  she  simply  presses  keys,  corresponding  to  the  number  of  the 
desired  subscriber,  and  the  impulses  are  transmitted  by  a  motor-driven 
machine,  which  must  complete  its  work  several  seconds  after  the  operator 
finishes  pressing  the  buttons,  a  guard  lamp  or  some  other  visual  signal 
should  be  used  in  connection  with  the  calling-device  keys,  which  will 
prevent  the  operator  from  attempting  to  set  up  a  second  call  on  the 
calling  device  before  the  machine  has  finished  transmitting  the  previous 
call. 

The  " B"  operator's  switchboard  may  be  situated  either  in  the  manual 
office  or  in  the  automatic  office.  If,  as  in  the  case  under  discussion, 
there  is  but  one  manual  office,  there  is  an  advantage  in  having  the  switch- 
board placed  in  it,  because  by  so  doing  all  operators  will  be  confined  to 
that  office,  and  no  rest  rooms  or  other  provisions  need  be  made  for  them 
at  the  automatic  office.  Furthermore,  it  will  be  easier  to  supply  reliefs 
and  to  enforce  good  discipline  if  all  the  operators  are  kept  in  one  office. 

Quite  often  when  a  manual  office  is  changed  to  automatic  equipment 
the  subscribers'  lines  are  distributed  among  an  automatic  main  office  and 
several  district  stations  surrounding  it.  Under  such  conditions  Plan  1 
for  handling  the  calls  from  the  manual  office  will  be  at  a  decided  dis- 
advantage, in  comparison  with  Plan  2;  because,  while  it  is  entirely 
practicable  to  use  the  multiple-switching  section  for  completing  connec- 
tions to  the  lines  in  the  main  automatic  office,  it  is  impracticable  to 
complete  connections  in  that-  manner  to  the  lines  terminating  in  the 
district  stations. 

If  Plan  2  is  used,  however,  the  "B"  operators,  by  means  of  their 
calling  devices,  will  complete  connections  to  district  station  subscribers 
just  as  easily  as  to  the  lines  connected  to  the  automatic  main  office. 
In  fact,  a  "Z?"  operator  need  not  know  to  which  office  a  line  she  is  calling 


336  AUTOMATIC  TELEPHONY 

is  connected.  If  the  traffic  between  the  "A"  operators  and  the  "B" 
operators  is  sufficiently  great  to  warrant  it,  the  use  of  an  automatic 
order-wire  distribution  system,  such  as  described  in  the  chapter  on 
automatic  traffic-distributor  equipment,  between  the  "A"  operators  and 
the  "B"  operators  should  be  considered. 

In  some  instances,  where  the  traffic  is  very  light,  each  "A  "  operator's 
position  has  been  equipped  with  a  calling  device.  When  this  is  done  the 
"A"  operator  by  means  of  her  calling  device  will,  herself,  complete 
connections  to  the  subscribers  in  the  various  automatic  offices. 

The  work  of  dialing  calls  over  automatic  trunks  slightly  increases  the 
work  of  the  "A  "  operator,  about  0.078  of  a  unit  call  per  additional  digit. 
Take  the  operator's  load  as  240  unit  calls  and  the  following  equating 
factors : 

Local  call 1 .  00 

Manual  trunk  call  (order  wire) 1 .  50 

Automatic  trunk  call  (4-digit) 1 .  65 

Automatic  trunk  call  (6-digit) 1 .  SI 

Assume  that  the  trunking  to  other  offices  is  60  per  cent.  Then  an 
operator  can  handle  75  local  calls  and  110  manual -trunked  calls.  To 
handle  this  number  of  calls,  if  the  trunk  calls  are  dialed  by  four  digits, 
would  require  1.069  "A"  operators,  and  if  the  calls  have  six  digits  it 
would  require  1.14  "A"  operators. 

To  dial  four-digit  calls  requires  about  seven  per  cent  more  "A" 
operators,  and  for  six-digit  calls  about  14  per  cent  more  operators. 

If  there  is  much  trunking,  the  "A"  operator's  busy-hour  load  is 
determined  by  the  congestion  on  order  wires  which  is  beyond  her  control. 
But  if  the  "A"  operator  can  dial  the  calls,  everything  is  within  her  own 
control. 

The  slight  additional  load  is  not  the  determining  factor  in  deciding 
on  a  plan  for  trunking  from  manual  to  automatic. 

Changing  a  Manual  System  of  More  than  Ten  Offices  to  an  Automatic 
System. — If  a  manual  system  including  more  then  ten  offices,  say  20 
for  example,  is  to  be  changed  over  to  full  automatic  equipment,  and  it  is 
not  considered  wise  to  attempt  to  change  all  of  it  at  one  time,  the  most 
practical  plan  is  to  take  the  first  step  by  dividing  the  system  into  not  to 
exceed  nine  districts.  In  full  automatic  practice  it  is  not  practicable  to 
have  more  than  nine  offices  of  the  first  magnitude,  since  but  nine  levels 
of  first-selector  banks  are  available  for  trunks  to  such  offices,  the  first 
level  being  reserved  for  special  service  trunks.  Therefore,  if  there  are 
more  than  nine  offices,  the  remaining  ones  must  be,  in  a  sense,  subsidiary 
to  the  nine  main  offices.  While  at  first  this  might  appear  to  be  a  dis- 
advantage it  is  really  an  advantage,  because  it  simplifies  the  trunking 
scheme  and  economizes  in  trunk  mileage,  as  explained  more  fully  in  the 
chapter  on  "Development  Studies." 


CUT-OVERS  AND  INTERCONNECTING  OFFICES  337 

Having  divided  up  the  system  under  discussion  so  that  there  are 
nine  main  offices,  some  of  which  have  one  or  more  satellites,  suppose 
that  it  is  decided  to  change  over  one  of  these  main  offices,  which  is  to  be 
called  the  "A"  office  and  which  is  to  have  three  district  stations  about 
it;  suppose  that  the  remaining  eight  offices  are  to  be  called  B,  F,  L,  H,  D, 
W,  X  and  Y,  respectively  These  letters  are  chosen  because  they  are 
least  likely  to  be  confused  one  with  another  when  spoken  over  the  tele- 
phone. Office  A  with  its  satellites  may  be  switched  to  full  automatic 
equipment  so  far  as  intercalling  between  the  various  subscribers  in  the 
"A"  district  is  concerned,  at  one  time,  just  as  if  there  were  no  other 
offices  in  the  system,  following  the  methods  described  in  the  opening 
paragraphs  of  this  chapter;  but  the  plans  for  handling  calls  out- 
going from  this  district  to  the  eight  other  districts  requires  further 
cosideration. 

Plan  1-A. — It  should  be  remembered  that  all  of  the  outgoing  calls 
originating  in  the  automatic  district  will  be  handled  by  trunks  which 
terminate  in  the  district  main  office,  because  the  subsidiary  offices  are 
sub-offices  only.  One  plan  for  trunking  calls  outgoing  from  "A"  would 
be  an  enlargement  of  the  scheme  explained  as  Plan  1  in  the  discussion  of 
method  for  changing  over  one  office  in  the  hypothetical  system  of  two 
manual  offices  only.  To  make  this  scheme  practicable,  subscribers' 
numbers  would  have  to  be  arranged  and  would  have  to  be  printed  in  the 
directory  in  such  a  way  that  the  subscribers  would  be  able  not  only  to 
distinguish  manual  numbers  from  automatic,  but  be  able  to  tell  to  which 
district  any  desired  manual  number  belonged.  This  could  be  done  by 
printing  the  word  "Manual"  in  front  of  all  manual  numbers. 

These  designations  would,  of  course,  have  no  significance  to  subscrib- 
ers to  manual  service;  but  to  subscribers  to  automatic  service  they  would 
mean  that  automatic  numbers  would  be  called  in  the  usual  automatic 
manner,  but  that  manual  numbers  must  be  secured  through  an  operator. 
In  every  case  the  present  exchange  names  should  be  replaced  by  a  prefix 
made  up  of  a  combination  of  two  letters,  such  as  A-B,  A-F,  etc.,  in  which 
the  first  letter  indicates  district  and  the  second  letter  the  office. 

For  carrying  out  this  plan  trunks  should  run  from  the  automatic 
main  office  to  each  of  the  eight  manual  district  centrals,  and  it  is 
recommended  that  all  automatic  subscribers  be  instructed,  when  a 
manual  number  is  desired,  to  call  the  first  letter  of  the  prefix  only,  and 
then  give  the  number  including  the  prefix  to  an  operator  who  would  thus 
be  secured.  These  orders  would  be  taken  by  "A"  operators  located 
at  each  of  the  half-dozen  central  district  offices.  When  such  a  call  was 
received  by  one  of  these  "A  "  operators,  she  would  handle  it  in  the  usual 
manner;  that  is,  if  the  party  desired  were  connected  to  her  office  she 
.would  complete  the  connection  in  the  multiple  in  front  of  her  without 
further  trunking.  If  the  party  desired  were  connected  to  one  of  the 
22 


338  AUTOMATIC  TELEPHONY 

subsidiary  offices  she  would  handle  the  connection  over  an  order  wire 
to  one  of  the  regular  " B"  operators  in  the  subsidiary  office. 

While  Plan  1-A  would  be  more  economical  in  trunk  mileage,  it  would 
entail  a  greater  expense  for  operator's  wages  and  will  give  slower  service 
to  subsidiary  offices  than  will  Plan  1-B. 

Plan  1-B. — The  only  object  in  having  the  calls  from  the  automatic 
district  to  the  various  subsidiary  manual  offices  pass  through  the  main 
offices,  instead  of  going  direct  to  the  subsidiary,  is  to  simplify  the  trunk- 
ing  plan  and  save  trunking  mileage.  Over  against  the  saving  in  trunk 
mileage,  however,  must  be  placed  the  increased  cost  of  operators'  hire. 
In  Plan  1-A  each  such  call  must  pass  through  an  "A"  operator  at  the 
district  central  office  to  a  "  B"  operator  at  the  subsidiary  office. 

It  is  possible  to  eliminate  the  work  of  one  operator,  by  having  trunks 
run  direct  from  the  automatic  district  main  office  second-selector  banks 
to  each  of  the  subsidiary  manual  offices,  and  there  terminate  in  "A" 
operators'  positions.  They  might  terminate  in  regular  line  jacks,  or  in 
cords  and  plugs.  The  latter  would  be  preferable  probably  in  the  larger 
manual  offices. 

Following  this  plan,  automatic  subscribers  would  be  instructed,  when 
calling  manual  numbers,  to  call  both  letters  of  the  prefix;  and  any  auto- 
matic subscriber  doing  so  would  secure  an  idle  trunk  direct  from  the 
automatic  main  office  to  an  operator  in  the  particular  manual  office  to 
which  the  party  he  desired  was  connected.  He  would  then  call  the  desired 
party's  number  to  the  operator,  either  including  or  omitting  the  prefix, 
and  she  would  complete  the  connection  by  plugging  into  the  proper  jacks 
in  the  multiple  in  front  of  her.  This  arrangement  will  give  faster  service 
than  that  which  is  secured  in  regular  manual  practice,  because  it  will  not 
require  more  then  two  seconds  for  an  automatic  subscriber  to  signal 
an  operator  in  the  particular  office  to  which  his  desired  party  belongs. 

Plan  1-C. — It  would  be  possible  to  have  outgoing  trunks  from  the 
"A"  district  main  office  terminate  in  first  selector  banks,  as  under 
Plan  1-A,  and  to  have  second  selector  switches  installed  at  each  of  the 
oight  other  main  offices  so  that  switching  of  calls  to  the  satellites  in 
each  district  would  be  done  in  the  main  central  office  of  the  district, 
but  this  would  scatter  the  automatic  equipment  about  so  much  and 
require  such  an  expensive  layout  for  power  plants  to  operate  the  auto- 
matic switches  in  the  various  offices  that  it  might  not  be  practicable. 

Generally,  Plan  1-B  would  be  the  preferable  one,  but  some  offices  may 
be  so  small,  01  be  situated  at  such  great  distances  from  other  offices,  that 
to  save  trunk  mileage  it  would  be  advisable  to  handle  the  calls  to  them 
under  Plan  J  A.  Of  course  if  Plan  1-A  and  Plan  1-B  were  both  used, 
it  would  be  necessary  for  the  automatic  subscribers  always  to  call  both 
letters  of  the  prefixes  instead  of  calling  one  letter  only  as  suggested  in 
Plan  1-A. 


CUT-OVERS  AND  INTERCONNECTING  OFFICES  339 

Plan  2-A. — This  plan  is  a  further  development  of  Plan  2  explained 
for  cutting  over  one  office  of  the  hypothetical  manual  system,  which 
had  two  offices  only.  It  will  be  remembered  that  the  plan  contemplated 
installation  in  the  manual  office  of  sufficient  selector  and  connector 
switches,  with  the  banks  of  the  connector  switches  multipled  to  the 
manual  switchboard,  to  enable  the  automatic  subscribers  to  call  the 
manual  subscribers  automatically.  While  the  installation  of  this  equip- 
ment might  be  warranted  in  some  of  the  larger  of  the  eight  main  offices, 
which  are  to  be  continued  as  manual  offices  for  a  considerable  length 
of  time,  it  is  not  likely  that  it  would  be  in  the  sub-offices  of  the  various 
districts;  consequently,  calls  to  sub-offices  would  have  to  be  handled 
in  accordance  with  Plan  1-C,  the  switching  being  done  to  the  "A" 
operators  at  any  sub-office  by  means  of  first  selectors  at  the  automatic 
main  office,  and  second  selectors  at  the  main  central  office  of  the  district 
to  which  the  sub-office  belonged. 

In  considering  this  plan  it  should  be  remembered  that  a  district 
which  is  operated  manually,  until  several,  or  a  majority,  of  the  other 
seven  districts  are  changed  over  to  automatic  equipment,  will  receive 
a  continually  growing  percentage  of  incoming  calls  from  automatic 
offices;  consequently,  if  the  way  in  which  the  various  districts  will  be 
changed  over  can  be  decided  upon  at  the  start,  the  installation  of  auto- 
matic switching  equipment  should  be  very  seriously  considered  for  the 
district  main  offices,  which  will  be  changed  over  at  the  latter  end  of  the 
schedule,  but  less  seriously  considered  for  those  which  will  be  changed 
over  earlier  in  the  schedule.  Very  little  of  the  automatic  equipment  in- 
installed  in  one  of  these  manual  offices  would  be  wasted,  because  it  would 
generally  be  the  same  as  that  used  in  a  full  automatic  office,  so  that  when 
it  was  no  longer  required  in  the  manual  office  it  could  be  removed  and 
used  for  additions  to  some  full  automatic  switchboard. 

Plan  3.  Semi-Automatic  Operation  During  the  Cut-over  Period. — 
A  third  plan  for  changing  over  a  multi-office  metropolitan  area  embodies 
the  use  of  semi-automatic  or  "Auto-manual"  equipment  during  the  cut- 
over  period.  While  this  increases  the  operating  cost  during  the  time 
of  the  cut-over,  it  simplifies  the  method  of  calling  from  the  subscriber's 
standpoint.  The  only  difference  between  this  plan  and  any  one  of  the 
plans  already  explained  would  be  the  omission  of  calling  devices  from 
the  subscribers'  instruments,  the  installation  of  operators'  keyboards 
with  special,  motor-driven  calling  devices  for  their  use  at  the  auto- 
matic central  office,  and  a  change  in  the  wiring  of  that  ofi  ce,  so  that  the 
outgoing  trunks  from  the  lineswitches  to  the  first  selectors  would  pass 
through  or  be  automatically  connected  with  the  positions  oF-he  operators. 

If  any  subscriber  to  the  semi-automatic  office  should-  desire  to  call, 
he  would  lift  his  receiver  from  the  switch-hook  and  secure  an  idle  operator, 
who,  if  he  wished  another  automatic  subscriber,  would  se'  up  the  con- 


340  AUTOMATIC  TELEPHONY 

ncction  for  him  automatically  by  operating  the  push-buttons  belonging 
to  one  of  her  machine-calling  devices. 

If  the  calling  party  should  wish  a  subscriber  connected  to  one  of  the 
manual  offices,  then  the  operator  would  have  to  do  just  what  a  subscriber 
would  have  to  do  if  trunking  schemes  like  Plans  1-A,  1-B,  or  2-A  were 
used.  In  other  words,  if  no  automatic  switches  were  installed  in  any  of 
the  manual  offices  and  Plan  1-A  were  used,  the  operator  would  secure 
another  operator,  in  the  main  central  office  of  the  manual  district  to  which 
a  desired  party  belonged,  by  calling  one  digit  only.  If  Plan  1-B  were 
used,  the  operator  would  call  two  digits  to  secure  another  operator.  If 
Plan  2-A  were  used,  he  would  set  up  the  connection  automatically, 
where  it  was  possible  to  do  so  and  where  arrangements  were  not  made  for 
automatic  calling,  she  would  secure  an  operator. 

One  objection  to  this,  from  a  traffic  engineer's  viewpoint,  is  that  a 
subscriber  would  be  required  to  give  his  order  twice  on  connections  requir- 
ing the  services  of  a  second  operator,  because  under  the  plan  explained, 
no  order  wires  arc  included  between  the  semi-automatic  operators  and 
the  operators  in  the  manual  offices. 

It  should  be  apparent  that  when  all  of  the  offices  in  the  various  dis- 
tricts had  been  changed  over  to  semi-automatic  service,  it  would  be  a 
simple  matter  to  change  the  subscribers'  stations,  one  at  a  time,  to  full 
automatic  service,  by  installing  a  calling  device  on  each  telephone,  and 
when  calling  devices  were  installed  on  all  of  the  telephones  connected 
to  a  given  lineswitch  group,  the  outgoing  trunks  from  that  group  might 
be  cut  out  of  the  operator's  switchboard  and  connected  straight  through 
to  secondary  lineswitches,  or  first  selector  switches,  if  secondary  line- 
switches  were  not  used.  At  the  same  time  the  subscribers  should  be 
provided  with  the  proper  instructions  for  full  automatic  calling,  and  be 
informed  that  thereafter  they  were  to  call  all  parties  without  the  aid  of 
operators. 

Plan  4. — This  plan  contemplates  proceeding  (as  under  Plan  1-A  or 
1-B)  by  providing  automatic  calling  from  the  beginning  for  intercon- 
nections between  subscribers  in  the  automatic  district,  but  to  arrange  it 
so  that  to  secure  manual  subscribers  the  automatic  subscribers  would 
call  the  same  number  in  every  instance;  for  example:  Finger  hole  9  on 
every  dial  might  be  labled  "manual"  and  the  subscribers'  directories  be 
arranged  so  that  the  word  "manual"  would  be  printed  at  the  side  of  the 
numbers  of  all  manual  telephones.  The  subscribers  would  be  instructed 
to  make  one  turn  of  the  dial  from  finger  hole  9  whenever  any  manual 
number  was  desired,  and  then  to  give  the  number  of  the  desired  party  to 
the  operator  who  would  respond.  It  is  apparent  that  the  trunks  would 
connect  to  the  ninth  level  of  the  first-selector  banks  in  the  main  auto- 
matic office.  The  operators  would  be  situated  at  a  special  trunking 
switchboard,  either  in  the  automatic  office  or,  preferably,  in  some  cen- 


CUT-OVERS  AND  INTERCONNECTING  OFFICES  341 

trally  located  manual  office.  These  operators  would  be  provided  with 
order  wires  and  trunks  to  each  of  the  various  manual  offices,  excepting 
small  outlying  offices  to  which  tandem  trunking  might  be  practiced. 
With  this  method  of  operating,  the  objection  mentioned  in  Plan  3  of 
having  the  automatic  subscribers  give  their  orders  twice  when  wishing 
manual  numbers  would  be  eliminated,  and  at  the  same  time  the  method  of 
securing  connections  would  be  so  simple  that  no  subscriber  should  have 
difficulty  in  understanding  how  to  proceed.  Furthermore,  if  the  cen- 
trally located  manual  office,  in  which  the  special  trunking  operators  were 
placed,  should  be  the  principal  office,  or  one  of  the  principal  offices  in  the 
business  district  of  the  city,  considerable  economy  would  be  secured  by 
making  the  multiple  of  the  manual  switchboard,  in  that  office,  available 
to  these  special  operators.  It  is  apparent  that  if  these  special  trunking 
operators  are  installed  in  a  centrally  located  manual  office,  that,  as  addi- 
tional outlying  offices  are  cut-over  to  automatic  equipment,  the  connec- 
tions from  them  to  the  remaining  manual  offices  may  be  very  satisfactorily 
handled  at  this  central  point.  Some  additions  to  the  force  and  equip- 
ment might  be  required  on  account  of  the  increased  traffic,  but  the 
economy  and  general  satisfaction  which  the  plan  would  give  would  be 
greater  than  if  the  trunking  operators  were  scattered  among  the  different 
automatic  offices. 

After  the  number  of  automatic  offices  changed  over  reached  a  certain 
point  the  number  of  trunking  operators  required  would  commence  to 
decrease,  because  of  the  large  number  of  connections  completed  auto- 
matically between  the  various  automatic  districts. 

In  considering  this  plan  for  any  city,  it  should  be  determined  whether 
it  may  not  be  made  a  very  economical  one  from  an  equipment  standpoint, 
by  using  portions  of  one,  or  more,  of  the  abandoned  switchboards  for 
constructing  or  adding  to  the  trunking  operator's  equipment. 

Plan  5.  The  Use  of  Call  Indicators. — This  scheme  has  all  the  operat- 
ing advantages  of  Plan  2,  page  330,  but  does  not  necessitate  the  large 
expenditure  for  second  selectors,  third  selectors  and  connectors  which  are 
required  in  the  manual  offices  as  outlined  in  that  plan. 

Let  us  assume  that  we  have  eight  offices  in  the  network,  each  having 
an  ultimate  of  10,000  lines.  The  first  four  offices  will  be  considered  as 
being  manually  operated,  while  the  last  four  will  be  full  automatic.  At 
each  automatic  office  the  second  level  of  the  first  selectors  leads  to  repeaters 
which  are  associated  with  trunks  terminating  on  special  equipment  at 
No.  2  manual  office.  The  third  level  leads  to  repeaters  which  are  asso- 
ciated with  trunks  to  the  No.  3  manual  office.  The  fourth  level  leads 
to  repeaters  which  are  associated  with  trunks  to  the  No.  4  manual  office. 
The  fifth  level  leads  to  repeaters  which  are  associated  with  trunks  to  the 
No.  5  manual  office.  The  6th,  7th,  8th  and  9th  levels  lead  to  second 
selectors  and  repeaters  as  is  common  practice  in  an  automatic  network. 


342 


A  UTOMA  TIC  TELEPIION Y 


At  the  No.  2  manual  office,  for  instance,  the  trunks  incoming  from 
the  second  level  of  the  first  selectors  in  the  automatic  offices  are  equipped 
with  rotary  lineswitches.  When  a  trunk  is  seized  at  an  automatic  office 
the  associated  rotary  lineswitch  extends  the  connection  to  an  idle  "  call- 
indicator  register. " 

The  "call  indicators"  will  be  installed  on  certain  positions  on  the  "B" 
boards  of  the  various  manual  offices.  Each  of  these  call  indicators  is 
made  up  of  40  small  lamps  under  a  plate  of  numbers  arranged  as  shown 


Fig.  277. — Call  indicator  lamp  bank. 

in  Fig.  277.  These  numbers  are  on  the  reverse  side  of  a  piece  of  ground 
glass  and  are  only  visible  when  the  associated  lamp  is  lighted.  The  key- 
shelf  equipment  on  a  call-indicator  position  is  shown  in  Fig.  278. 

Should  an  automatic  subscriber  remove  his  receiver  and  dial  the 
figure  2,  his  connection  will  be  extended  to  a  plug  ending  trunk  on  a  B 
position  and  a  rotary  lineswitch  at  the  No.  2  manual  office.  The  rotary 
lineswitch  will  automatically  connect  the  trunk  with  a  vacant  call- 
indicator  register,  of  which  there  are  eight  for  each  call-indicator  "5" 

Number 

Pbtes 

Plugs 


ooooooooooooooooooooooooooooooooooooooooooooooc/-'' 
oooooooooooooooooooooooooooooooooooooooooooooooo*' 

!:'  bcOOOOOOOCOOOCOOOCXDOOOCODOCXDOOOCOOOCOOCOOOOOOOdo''' 
V  OOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOCXD^' 


Designation 
Strips 


00000000;—-Se''t*/:'  Pilots 
OOOOOOOCK-  Cut-in  Keys 


Assignment 
'lamps 

Disconnect" 
Lamps 


Fig.  278. — Diagram  of  keyshelf  with  call  indicator. 

position.  As  the  subscriber  dials  the  remaining  digits  of  the  number, 
4-0-7-3  for  example,  the  register  records  this  number  and  as  the  last 
number  is  recorded,  the  "set-up  pilot"  of  the  register  which  is  being  used 
lights  up,  as  also  does  the  "assignment  lamp"  associated  with  the  trunk 
on  which  the  call  is  being  made,  and  at  the  same  time  the  numbers 
4-0-7-3  will  appear  on  the  call  indicator,  as  is  indicated  by  the  heavy 
type  in  Fig.  238.  The  operator  now  completes  the  connection  by  insert- 
ing the  plug  which  is  associated  with  the  lighted  assignment  lamp,  into 
the  multiple  jack  4073,  following  which  the  number  displayed  on  the  call 


CUT-OVERS  AND  INTERCONNECTING  OFFICES  343 

indicator  disappears,  the  set-up  pilot  and  the  assignment  lamp  going  out 
at  the  same  time.  If,  in  the  meantime,  another  call  has  come  into  the 
position  the  same  cycle  of  operation  is  repeated. 

In  other  words,  the  rotary  lineswitch  extends  each  trunk  to  a  register 
which  is  not  busy,  and  the  remaining  digits  dialed  by  the  subscriber  are 
recorded  on  the  register  he  obtains.  The  registered  number  is  displayed 
on  the  call  indicator,  and  is  cleared  as  the  calling  plug  is  inserted  in  the 
jack  of  the  line  wanted. 

The  operator  tests  for  busy  in  the  usual  way,  and  if  the  called  line  is 
busy  the  calling  plug  will  be  inserted  into  a  "busy  back"  jack  as  is 
customary  in  manual-to-manual  practice. 

When  employing  this  scheme  no  distinction  is  made  in  the  directory 
between  manual  and  automatic  numbers,  as  from  the  automatic  sub- 
scribers' viewpoint  the  operation  of  calling  is  the  same  in  either  case. 

If  at  any  time  in  the  future  it  is  desired  to  convert  a  manual  office  to 
automatic,  it  is  only  necessary  to  replace  the  call-register  apparatus  by 
incoming  second  selectors,  third  selectors  and  connectors. 

Calls  from  Manual  to  Automatic. — Connections  from  the  various 
manual  offices  to  an  "A  "  automatic  district  should  be  made  by  means  of 
the  requisite  number  of  "B"  operators  working  at  a  special  semi-auto- 
matic switchboard  as  explained  in  the  outline  of  methods  for  cutting  over 
one  office  in  the  hypothetical  system  of  two  manual  offices  only.  It  is 
stated,  however,  in  that  explanation,  that  it  might  be  a  question  whether 
the  "B"  operator's  switchboard  should  be  located  in  the  new  automatic 
office  or  in  the  remaining  manual  office. 

In  the  example,  now  under  discussion,  the  "B"  switchboard  could  be 
put  either  in  the  automatic  main  office,  or  in  a  centrally  located  manual 
office.  Which  plan  would  be  preferable  would  depend  upon  circum- 
stances. If  the  first  automatic  district  cut-over  should  be  an  outlying 
one,  a  "5"  board  installed  there  would  probably  be  of  no  use  in  cutting 
over  succeeding  districts;  whereas,  if  it  should  be  placed  in  a  centrally 
located  office,  it  could  be  used  with  necessary  additions  for  handling  all 
of  the  trunks  from  manual  offices  to  automatic  offices  until  the  entire 
cut-over  was  completed. 

Doubtless  under  some  conditions  the  plan  of  putting  semi-automatic 
operators'  switchboards  in  each  of  the  main  automatic  offices  might  be  the 
most  economical  of  trunk  mileage,  and  in  some  cases  be  the  best  from  a 
transmission  standpoint;  but  it  would  seem  that  in  any  instance,  the  plan 
of  using  one  centrally  located  switchboard  would  be  the  most  economical 
from  the  equipment  and  operating  standpoints. 

Another  attractive  feature  of  the  combined  centrally  located  board  is 
that  it  makes  possible  the  introduction  of  traffic  distributor  equipment,  on 
either  the  order  wires  or  the  trunks  incoming  to  the  semi-automatic  "B" 
operators  from  the  "A"  operators  in  the  various  manual  offices,  if  first 


344  AUTOMATIC  TELEPHONY 

selector  switches  should  be  installed  either  in  the  building  with  the  semi- 
automatic switchboard,  or  in  a  nearby  automatic  office,  making  it  un- 
necessary for  the  "B"  operators  to  have  direct  trunks  to  the  different 
automatic  main  offices.  In  other  words,  if  first  selector  switches  were 
available  to  them,  any  "B"  operator  would  be  able  to  call  any  automatic 
subscriber,  in  any  office,  by  using  her  push-button  calling  device.  This 
makes  it  practicable  to  introduce  traffic  distributor  equipment  of  a 
character  similar  to  that  described  in  the  chapter  especially  devoted  to 
apparatus  of  that  kind  for  distributing  the  load  among  the  "B"  operators. 

Line  switch  Sub -offices  in  Connection  with  the  Manual  Offices. — 
It  would  not  be  necessary  to  wait  until  the  main  office  in  each  manual  dis- 
trict was  changed  over  to  automatic  equipment  before  changing  over  one 
or  more  of  that  main  office's  satellites  to  lineswitch  sub-office  apparatus, 
such  as  described  in  the  chapter  on  semi-auto  sub-offices.  In  fact  not 
only  might  some  of  the  existing  outlying  manual  offices  be  changed  over 
to  equipment  of  that  character,  but  if  a  congestion  should  occur  at  any 
point  which  would  appear  to  require  the  installation  of  additional  equip- 
ment or  cable,  for  relief,  a  lineswitch  sub-office  should  be  very  seriously 
considered  before  any  other  means  is  decided  upon. 

Branch  Offices. — To  make  the  discussions  of  various  methods  for 
cutting  over  as  clear  as  possible,  the  use  of  "branch"  automatic  offices 
has  not  been  considered.  Such  offices  have  their  place,  as  explained  at 
considerable  length  in  the  chapter  on  "Development  Studies." 


CHAPTER  XV 

POWER  PLANT,  SUPERVISORY  AND  TESTING  EQUIPMENT,  AND 

CIRCUITS 

Power  Plants. — A  power  plant  of  an  up-to-date  automatic  switch- 
board central  office  generally  consists  of  one  storage  battery,  two  ringing 
equipments,  two  battery  charging  equipments,  one  power  switchboard, 
and  one  supervisory  cabinet. 

Storage  Battery. — If  the  equipment  is  designed  to  operate  on  a  normal 
difference  of  potential  of  46  volts,  the  battery  consists  of  25  cells  with 
seven  counter  electromotive  force  cells.  The  latter  are  used  to  keep  the 
pressure  between  46  and  52  volts  at  the  bus  bars.  Except  in  small 
offices,  it  is  the  practice  to  install  each  cell  in  a  lead-lined  wooden  tank 
because  glass  cracks  too  easily.  The  cells  are  mounted  on  battery  racks 
from  which  each  is  insulated  by  four  glass  insulators.  The  racks  are 
insulated  from  the  floor  by  glass  insulators  resting  on  vitrified  brick. 

Since  a  number  of  standard  books  and  other  publications  are  avail- 
able which  give  full  instructions  concerning  the  installation  and  care 
of  storage  batteries,  space  will  not  be  occupied  here  by  an  attempt  to 
cover  those  subjects. 

Charging  Machines. — Two  charging  equipments  are  usually  installed: 
one  consisting  of  a  motor-generator  set,  or  rectifier,  and  the  reserve  or 
emergency  set  either  a  gas  or  gasoline  engine  direct  connected  to  a  charg- 
ing generator.  A  mercury  arc  rectifier  has  a  higher  efficiency  than  any 
other  type  of  charging  outfit  for  use  in  deriving  power  from  an  alternating- 
current  supply,  and  is,  therefore,  generally  used  for  offices  which  will  not  re- 
quire an  ultimate  charging  rate  to  exceed  50  amp.  Two  mercury  arc 
rectifiers  may  be  used  in  multiple  to  supply  100  amp.,  but  it  is  the  general 
practice  to  use  motor-generator  charging  machines  for  offices  requiring 
a  charging  rate  higher  than  the  output  of  one  rectifier. 

The  rectifier  apparatus  is  mounted  on  a  slate,  or  marble,  panel  to 
match  the  balance  of  the  power  switchboard  of  which  it  becomes  a  part. 
Figure  279  is  a  front  view  of  such  a  switchboard  on  which  the  second 
panel  from  the  right  is  the  rectifier  panel.  The  circuits  will  be'  described 
further  on  in  this  chapter. 

Where  direct  current  of  110  volts  is  available,  charging  may  be  done 
directly  from  the  supply  mains  through  a  suitable  resistance,  but  on 
account  of  danger  of  making  the  talking  circuits  noisy,  this  method  is 
rarely  practised,  except  for  small  private  exchanges. 

345 


346 


AUTOMATIC  TELEPHONY 


Motor  generators  for  charging  sets  are  preferably  of  special  design, 
in  that  the  generator  commutators  have  an  extra  large  number  of  bars, 
and  the  armature  is  generally  of  a  "smooth  core"  type  in  order  to  reduce 
the  danger  of  noise  on  talking  circuits.  If  a  smooth  core  armature  is  not 
used,  the  charging  circuit  is  passed  through  an  impedance  coil  to  "  smooth 
out"  the  current. 

The  generator  is  generally  compound-wound,  but  arranged  so  that  it 
be  changed  to  a  shunt-wound  machine  by  switching  one  of  the  armature 
lead  fuses  to  another  clip  on  the  terminal  block,  which  is  mounted  on  the 
frame  of  the  machine.  It  is  ordinarily  used  as  a  shunt  machine  and  the 
compounding  is  only  used  in  case  of  emergency,  when  the  battery  is 


Fig.   279. — Typical  power  and  battery  switchboard. 

disabled  and  the  dynamo  must  take  the  switchboard  load  direct.  A 
common  specification  for  the  compound  winding  requires  that  it  shall  be 
such  that  under  operating  conditions  it  will  automatically  regulate  the 
voltage,  so  that  it  will  not  drop  lower  than  46,  nor  rise  higher  than  52, 
while  the  current  delivered  by  the  machine  varies  from  its  full-rated 
amount  to  1/15  of  the  full-rated  amount. 

The  motor  and  dynamo  of  motor-generators  are  direct  connected  and 
mounted  on  a  common  sub-base.  The  motor  may  be  of  any  standard 
design. 

Ringing  Current. — Harmonic  converters  (pole  changer  sets)  and 
motor  generators  are  used  for  ringing,  one  of  the  latter  being  shown  at  the 


POWER  PLANT,  SUPERVISORY  AND  TESTING  EQUIPMENT    347 

right  of  Fig.  279.  The  motor  generator  is  usually  equipped  with  sets  of 
springs  (seen  in  the  view  referred  to  above)  designed  to  furnish  the  make 
and  break  contacts  for  the  generator  control  relays  and  to  distribute 
the  switchboard  load  in  such  a  way  that  the  generator  will  supply  ringing 
current  to  the  various  switchboard  sections  in  succession,  and  not  be 
required  to  furnish  the  ringing  current  needed  throughout  the  entire 
switchboard  at  one  time.  In  addition  to  the  ringing  springs  these  motor 
generator  outfits  carry  busy,  dial  tone  and  howler  attachments.  Some- 
times the  ringing  machine  used  regularly  is  operated  by  the  commercial 
power  circuit,  while  the  reserve  machine  is  a  dynamotor  driven  from  the 
exchange   storage   battery.     With  this   arrangement,   a  failure   of  the 


Fig.  280. — Power  switchboard  and  charging  machine. 

commercial  source  of  power  supply  for  a  few  hours  will  not  disable  the 
telephone  plant.  The  reserve  ringing  apparatus,  in  the  plant  in  which 
the  photograph  reproduced  in  Fig.  279  was  taken,  is  a  harmonic-con- 
verter outfit;  in  fact,  two  harmonic-converter  ringing  outfits  are  installed, 
each  consisting  of  a  pole  changer,  a  suitable  transformer,  for  furnishing 
33.3  cycle  ringing  current,  and  a  device  for  furnishing  the  interrupted 
buzz,  used  as  a  busy  signal. 

When  harmonic  converters  are  used  it  is  customary  to  provide  sole- 
noid ringing  interrupters  to  feed  interrupted  ground  to  the  generator 
control  relays.  Two  of  these  interrupters  are  shown  at  the  extreme 
upper  right-hand  side  of  the  power  board  in  Fig.  279. 

This  ringing  interrupter  (Fig.  281)  consists  of  a  solenoid  which,  by 


:;is 


A  UTOMA  TIC  TELEPIION  Y 


means  of  its  plunger,  controls  a  rod  carrying  a  pair  of  wipers,  which  are 
arranged  to  move  back  and  forth  over  two  rows  of  seven  contacts  each. 
When  the  solenoid  has  drawn  in  its  plunger,  thereby  raising  each  wiper 
to  the  top  contact  in  its  row,  the  circuits  are  such  that  the  plunger  is  then 
allowed  to  return  to  normal  position  by  the  force  of  gravity.  The  speed 
at  which  it  returns  is  controlled  by  a  piston,  mounted  on  the  lower  end  of 
the  rod,  which  moves  through  a  cylinder  filled  with  oil.  The  circuits  of 
this  device  will  be  explained  in  detail  further  on.  The  purpose  of  it  is 
to  operate  the  relays  which  supply  ringing  current  from  the  harmonic 
converters  to  the  automatic  switchboard,  a  section  at  a  time.  A  switch- 
board may  be  divided  into  five  sections,  and 
as  the  wipers,  when  descending,  pass  over  the 
second  pair  of  contacts  from  the  top,  ringing 
current  is  supplied  to  the  first  section  and  the 
ringing  relays  of  the  connectors  in  ringing 
position  in  that  section  are  operated.  When 
the  wipers  pass  to  the  third  pair  of  contacts, 
ringing  current  is  supplied  to  the  second  sec- 
tion and  the  ringing  relays  of  the  connectors 
in  ringing  position  in  that  section  are  operated, 
etc.  This  device  may  be  used  for  doing  the 
interrupting,  where  dynamotors  or  motor 
generators  supply  the  ringing  current  also. 
It  costs  less,  and  is  less  expensive  to  install, 
maintain  and  operate  than  the  spring  and  cam 
equipment  operated  by  the  rotary  machines. 

Power  Switchboard. — A  power  switch- 
board consists  of  the  required  number  of  slate 
or  marble  panels,  mounted  upon  suitable  angle 
iron  frames.  On  these  panels  are  mounted 
the  necessary  instruments  for  operating  and 
controlling  the  various  battery  charge  and 
discharge  circuits,  the  circuits  of  the  ringing 
machines,  etc.  A  voltmeter  is  provided, 
which  ordinarily  has  a  scale  reading  from  0  to  75  volts.  If  district 
stations  are  used,  which  are  to  be  charged  by  using  the  120-volt  circuit 
of  a  rectifier  installed  at  the  central  office,  the  voltmeter  should  read  up 
to  120  volts. 

An  ammeter  with  center  zero  is  also  installed.  Circuits,  to  be 
explained  later  on,  will  show  the  ammeter  so  connected  that  when  the 
charging  machine  is  in  operation  it  will  show  the  net  amount  of  current 
which  the  battery  is  receiving;  and  when  the  charging  is  not  in  operation, 
it  will  show  the  amount  of  current  which  the  battery  is  delivering  to  the 
switchboard.     Furthermore,   shunts  are   provided  so  that  when  either 


Fig. 


281. — Solenoid     ringing 
current  interrupter. 


POWER  PLANT,  SUPERVISORY  AND  TESTING  EQUIPMENT     349 

charging  machine  is  in  operation  the  current  delivered  by  it  may  be 
measured. 

Instrument  switches  are  used  for  switching  the  Volt  and  ammeters  in 
connection  with  the  various  circuits.  The  hand  wheels  of  these  instru- 
ments are  shown  directly  underneath  the  meters  in  each  of  the  views  of  a 
power  board.     (See  Figs.  279  and  280.) 

A  "Reversite"  and  overload  circuit  breaker  is  installed  for  controlling 
each  charging  circuit.  One  of  these  is  shown  at  the  bottom  of  the  left- 
hand  panel  of  Fig.  279. 

At  the  bottom  of  the  second  panel  from  the  left  is  seen  an  eight-point 
counter-e.m.f.  cell  switch,  which  is  provided  on  each  power  board  for 
regulating  the  voltage  on  the  main  discharge  bus  bars,  by  switching  in 
and  out  the  counter-e.m.f.  cells  already  referred  to.  Each  power  board 
is  generally  equipped  with  a  high-  and  low-voltage  alarm  relay,  which 
rings  a  bell  when  the  voltage  on  the  main  switchboard  feeders  drops 
below  46,  or  rises  higher  than  52. 

Typical  Power-board  Circuits. — In  Fig.  282  is  shown  a  typical  power- 
plant  wiring  diagram.  In  the  lower  left-hand  corner  are  the  connections 
of  the  storage  battery,  the  counter-e.m.f.  cells  and  the  counter-e.m.f.  cell 
switch.  It  will  be  noted  that  from  the  batttery  one  pair  of  feeders  leads 
to  the  power-distribution  panel,  from  which  go  the  power-supply  circuits 
to  the  various  sections  of  apparatus.  This  pair  of  bus  bars  supplies 
current  also  to  one  of  the  two  rotary  ringing  machines.  The  other  pair 
of  bus  bars  leads  to  the  two  charging  outfits,  one  of  which  consists  of  a 
motor  generator  operated  on  a  220-volt,  60-cycle,  single-phase,  commercial 
power  circuit. 

The  reserve  set  is  a  four-cylinder  gas  engine  (1,  2,  3,  4)  direct  con- 
nected to  a  charging  machine  (No.  2).  The  mains  from  the  charging 
generator  pass  through  a  "Reversite"  and  overload  circuit-breaker.  A 
voltmeter  and  its  switch  are  shown  in  the  upper  left-hand  corner  of  the 
diagram,  but  for  simplicity's  sake  the  wiring  is  not  given  in  full.  At 
various  points  of  the  power-board  wiring,  however,  are  seen  arrowheads, 
with  such  designations  as  V.S.-3,  F.&.-4,  etc.,  which  indicate  the  points  at 
which  the  various  voltmeter  switch  connections  are  made.  Beneath 
the  voltmeter  are  the  ammeter  circuits.  One  of  the  ammeter  shunts 
is  used  for  measuring  the  current  of  each  charging  machine,  and  the  third 
is  used  for  measuring  the  current  flow  from  the  battery  to  the  automatic 
switchboard. 

In  Fig.  283  is  shown  the  power  switchboard  wiring  digram  of  a  main 
office  with  three  sub-offices.  In  these  offices  the  ringing  current  and 
busy-signal  current  are  furnished  by  harmonic  converters.  The  charging 
is  done  by  means  of  rectifiers.  As  indicated  in  the  lower  right-hand 
corner  of  the  diagram,  each  rectifier  is  connected  to  a  220-volt,  60-cycle, 
single-phase  commercial  power  circuit,  by  means  of  an  insulating  trans- 


350 


AUTOMATIC  TELEPHONY 


?  O//    QODU  4UI 


POWER  PLANT,  SUPERVISORY  AND  TESTING  EQUIPMENT    351 


former.  The  use  of  such  a  transformer  is  common  practice,  to  avoid 
danger  from  a  punctured  transformer  on  the  commercial  power  circuit. 
A  reactance  coil  is  connected  between  the  rectifier  and  the  charging  mains 
to  eliminate  any  noise  on  talking  circuits. 

As  indicated,  the  batteries  at  each  of  the  sub-offices  may  be  charged 
by  means  of  the  rectifiers  at  the  main  office  through  a  rheostat  placed  in 
the  negative  battery  feeder  of  each  sub-office.  No  positive  battery 
feeder  is  provided,  because  the  connection  for  the  positive  side  is  made 
through  the  earth,  reinforced,  quite  often,  by  the  sheath  of  the  cable 
supplying  the  trunks  to  each  sub-office. 


Rectifier *Z 


Pect, 


i 


React 
Coil 


.16  6  Busbar 
►33.3-     ' 
-50~       « 
:66.6       » 


In  Sup'y  Cab. 


110  V.  Lamps, ; 
-To  Busu Busbar]  J".   °n  Board  ' 
--TOR  J      .      }%%< 


to  bust/1     -"arm?l      Harm?2 
Mach. 


Ammeter  Voltmeter 

Fig.  283. — Typical  power  plant  circuit. 

Battery,  Ringing,  and  Tone  Distribution  Scheme. — In  Fig.  284  is 

shown  a  power  distribution  scheme  which  should  be  studied  in  connection 
with  Fig.  282.  The  positive  and  negative  terminals  of  the  battery  are 
carried  to  bus  bars  on  the  main  power  switchboard.  The  positive  is 
grounded  and  the  negative  is  carried  through  fuses  to  the  separate  cir- 
cuits. There  is  one  pair  provided  to  feed  battery  to  each  group  consist- 
ing of  ten  line  switchboards.  This  pair  runs  to  the  nearest  board  of  a 
group,  and  terminates  upon  the  bus  bars  of  a  small  auxiliary  power-board 
at  the  top  of  the  lineswitch  upright.  This  pair  of  bus  bars  is  strapped 
to  a  similar  pair  of  bus  bars  on  the  next  line  switchboard  of  the  group; 
the  strapping  is  continued  until  all  the  boards  in  the  .group  have  been 
reached. 


352 


AUTOMATIC  TELEPHONY 


From  each  pair  of  bus  bars  the  current  is  distributed  to  the  various 
parts  of  the  associated  lineswitch  board.  Each  connector  switch  receives 
its  battery  from  the  bus  bar  over  an  individual  pair.     This  pair  feeds 


1^  ^ 
****** 

6^  cc^ 


S4IOAQf 


everything  in  the  switch,  excepting  the  release  magnet,  which  draws  its 
current  through  a  release  signal  relay  which  is  common  to  all  the  con- 
nectors on  this  lineswitch  board. 

Each  master  switch  has  its  own  pair  of  wires  supplying  positive  and 


POWER  PLANT,  SUPERVISORY  AND  TESTING  EQUIPMENT     353 

negative  battery.  In  addition,  supervised  positive  battery  comes  through 
a  one-half-ohm  relay  and  the  supervisory  ground  bus  bar.  One  master 
switch  signal  wire  is  provided  for  each  master  switch;  this  operates  on 
alarm  if  the  master  switch  should  stick  between  notches  on  the  escape- 
ment sector. 

One  pair  of  positive  and  negative  battery  wires  are  provided  for  each 
section  of  25  lineswitches.  The  B.C.O.  coils  receive  their  current  through 
on  fuse  for  the  entire  100  lineswitches.  There  is  one  supervised  ground 
wire  for  each  section  of  lineswitches,  which  supplies  positive  battery  to 
the  line  relay  contact  for  operating  the  pull-down  coil. 

Power  to  Selector  Boards. — There  is  one  pair  of  battery  feed  wires 
for  each  kind  of  selector.  Thus,  there  will  usually  be  one  pair  for  the 
first  selectors  (even  though  there  may  be  several  boards  of  first  selectors), 
another  pair  for  the  second  selectors,  another  pair  for  the  third  selectors, 
etc. 

Taking  the  first  selectors  as  an  example,  the  battery  pair  runs  along 
the  tops  of  adjacent  selector  boards  with  a  tap  from  each  wire  leading 
down  to  auxiliary  bus  bars  placed  on  a  small  power  board  which  is 
mounted  just  above  the  door  leading  into  the  selector  board. 

The  distribution  is  divided  into  ten  parts;  the  low  side  of  the  board 
and  the  high  side  of  the  board.  There  are  five  (sometimes  six)  shelves 
on  each  side  or  bay,  each  shelf  carrying  20  selectors.  There  is  one  pair 
of  wires  carrying  positive  and  negative  battery  to  each  shelf;  the  negative 
battery  is  fused,  the  positive  is  fed  direct.  After  reaching  the  shelf  the 
battery  feed  is  multipled  from  switch  to  switch.  There  is  a  single  fuse 
on  the  power  panel  which  feeds  the  signal  relays. 

The  line  relay  of  a  selector  is  supplied  with  two  sources  of  positive 
battery.  One  conductor  passes  from  the  positive  bus  bar  through  one 
winding  of  an  induction  coil.  When  the  switch  is  seized,  the  supervisory 
relay  which  is  in  series  with  the  release  relay  causes  a  trunk  tone  to  be  sent 
through  the  primary  winding  of  the  same  induction  coil,  so  that  the 
calling  subscriber  will  hear  the  tone  and  be  notified  that  he  has  secured  a 
selector,  and  the  circuit  is  ready  for  dialing.  The  other  source  of  positive 
battery  will  be  discussed  later.  The  release  magnet  obtains  its  negative 
battery  through  the  release-signal  relay,  which  is  for  the  purpose  of 
indicating  when  the  release  magnet  draws  current  longer  than  is  safe. 

The  second  and  third  selectors  are  wired  in  a  similar  manner,  except 
that  they  are  not  provided  with  a  trunk  tone. 

Ringing  Current  Distribution.— Ringing  current  for  straight-line 
work  is  illustrated  in  this  figure.  If  this  is  understood  the  slight  varia- 
tions found  in  party-line  work  can  easily  be  mastered. 

The  alternating-current  generator  has  one  terminal  attached  to  a 
separate  positive  bus  bar  connected  to  earth  and  the  other  terminal  to  a 
generator  bus  bar  equipped  with  fuse  distribution.     Across  the  two  bus 


354  AUTOMATIC  TELEPHONY 

bars  is  a  generator-alarm  relay  which  is  normally  energized.  If  the 
generator  fails,  the  falling  block  of  the  relay  sounds  an  alarm.  Current 
for  the  wire  chief's  desk,  etc.,  is  supplied  through  a  pair  of  wires  equipped 
with  a  lamp  resistance  in  the  fused  lead. 

The  ringing  current  for  the  automatic  board  comes  through  a  fuse 
and  is  distributed  to  the  front  contacts  of  several  generator-control 
relays.  Each  generator-control  relay  is  operated  by  an  interrupter,  so  as 
to  supply  ringing  current  at  intervals,  between  which  the  ringing  wire 
leading  to  the  connectors  is  grounded.  The  purpose  of  this  was  described 
when  discussing  the  connector  switch. 

There  is  one  generator-control  relay  for  each  group  of  lineswitch 
boards.  The  size  of  the  group  may  vary  from  five  to  ten  boards.  The 
ringing  current  is  tapped  off  at  each  lineswitch  board  through  a  lamp 
resistance,  after  which  it  is  multipled  directly  to  the  jacks  of  the 
connectors. 

There  is  one  interrupter-start  relay  for  every  five  groups,  this  relay 
feeds  negative  battery  to  the  generator-control  relays. 

The  interrupter-start  relay  is  controlled  by  a  wire  which  is  multipled 
to  all  five  groups  and  distributed  in  each  lineswitch  board  to  the  jacks 
of  all  the  connectors.  The  interrupter  which  controls  the  generator-con- 
trol relays  is  arranged  so  as  to  pull  up  one  relay  at  a  time.  This  spreads 
out  the  ringing  so  that  the  generator  is  supplying  current  to  only  one 
group  at  a  time. 

Busy  and  Trunk  Tone  Distribution. — As  shown  in  the  figure,  the  busy 
tone  is  generated  by  the  feeding  of  battery  current  through  a  high-speed 
commutator  to  an  induction  coil.  There  is  also  an  interrupter  which 
breaks  up  the  tone  into  suitable  pulsations.  There  is  one  busy-tone 
wire  leading  from  the  power  board  to  each  group  of  lineswitch  boards. 
The  top  taken  off  at  each  lineswitch  board  passes  through  a  lamp  resist- 
ance and  a  condenser,  and  is  then  multipled  to  the  jacks  of  the  con- 
nectors. 

The  overflow  busy  tone  is  taken  from  the  regular  busy-tone  wire  at 
the  power  board  by  means  of  an  induction  coil.  This  is  done  so  as  to 
secure  connection  to  the  positive  bus  bar.  It  is  supplied  by  means  of  one 
wire  for  each  group  of  selector  boards.  At  each  selector  board  the  over- 
flow busy-tone  wire  divides  into  two  parts — one  for  the  high  side,  the 
other  for  the  low  side.  Each  side  again  divides  into  one  wire  per  shelf, 
which  is  multipled  to  the  selector  jacks.  This  is  the  second  kind  of 
ground  furnished  to  the  line  relay  and  comes  into  use  when  all  trunks  on 
a  given  level  are  busy  and  the  selector  rotates  of  the  bank.  The  second 
and  third  selectors  are  also  provided  with  the  overflow  busy  tone. 

The  trunk  tone  is  generated  in  a  very  similar  way  to  the  generation  of 
the  busy  tone.  This  is  distributed  to  first  selectors  only.  The  trunk- 
tone  wire  is  tapped  off  at  each  selector  board  and  distributed  through  two 


POWER  PLANT,  SUPERVISORY  AND  TESTING  EQUIPMENT     355 

condensers,  one  for  each  bay.  There  is  one  wire  for  each  mechanical 
shelf  of  20  switches.  It  runs  to  the  primary  of  an  induction  coil  so  that 
when  the  supervisory  relay  is  operated,  it  will  put  this  tone  on  the  regular 
positive  battery  feed  for  the  line  relays. 

Sub-office  Power  Equipment  and  Circuits.— The  battery  of  a  sub- 
office  not  more  than  a  mile  from  its  main  exchange  may  be  floated  on  the 
main  exchange  battery  of  25  cells  through  cable  pairs.  If  the  distance 
to  the  sub-office  is  too  great,  or  the  spare  cable  conductors  not  low 
enough  in  resistance,  a  120-volt  rectifier  installed  atthe  central  office, 
as  indicated  in  Fig.  284,  may  be  used.     Where  this  plan  is  not  pract- 


^iNo.29  Cutoff 
•5 


V)No.2Q  Charge 


Rectifier 
Tube 


Fig.   285. — Circuits  of  a  mercury  arc  rectifier  charging  outfit  for  a  sub-office. 


icable,  it  is  common  practice  to  charge  the  sub-office  batteries  by  means 
of  connections  to  the  commercial  source  of  power  supply,  tapped  into 
the  sub-office  for  the  purpose.  If  110-volt  direct  current  is  available, 
the  sub-office  battery  may  be  charged  directly  through  a  rheostat.  If 
alternating  current  is  available,  it  is  customary  to  use  a  mercury  arc 
rectifier. 

The  circuit  of  a  typical  sub-office  rectifier  equipment  is  shown  in  Fig. 
285.  Relays  are  arranged  to  enable  the  wire  chief  at  the  central  office 
automatically  to  switch  the  sub-office  charging  apparatus  on  or  off,  as 
desired,  by  calling  certain  numbers  on  his  test-distributor  switch,  using  a 
calling  device  on  his  desk.  Circuits  of  these  relays  are  indicated  in  Fig. 
285.     To  charge,  for  example,  the  wire  chief  calls  20,  whereupon  the 


356 


A  U  TOM  A  TIC  TELEPHOS  Y 


private  wiper  of  the  test-distributor  switch  connects  the  wire  marked 
"charge"  to  earth.  This  operates  the  1300-ohm  relay,  which  closes 
circuit  from  earth  through  the  two  18-ohm  coils  of  the  electromagnet  to 
negative'  battery.  This  electromagnet  attracts  its  armature  which  closes 
the  power-supply  circuit,  as  shown.  At  the  same  time  that  the  electro- 
magnet closes  the  switch,  it  breaks  its  own  circuit  and  the  switch  lever  is 
locked  mechanically  by  the  armature  of  the  310-ohm  relay.  When  the 
circuit  through  the  primary  of  the  transformer  is  closed,  the  secondary 
potential  is  brought  to  bear  upon  the  rectifier  tube,  and  at  the  same  time 
the  shaking  coil  shakes  the  tube,  starting  the  mercury  vapor  arc,  between 


~*   110  VAC. 


Across  Busbars 


Bond  to  Cable 
Sheath  -. 


Slow  Relay 


jO-IOArr.o. 


^l  oca  I  Ground 


To  Sub.  -  Ex .  Test  Keys 
&  Phone 


Fig.  2ISG.- 


-Circuit  of  sub-office  power  plant    arranged  for  remote  control  of  charging  ap- 
paratus and  automatic  control  of  heating  apparatus. 


the  starting  anode  and  the  cathode.  The  arc  is  taken  up  by  one  or  other 
of  the  regular  anodes  and  the  solenoid  switch  is  then  actuated,  which 
closes  the  circuit  from  the  cathode,  through  the  ammeter  A,  circuit- 
breaker,  the  solenoid  switch  coil  and  the  storage  battery  direct  to  the 
middle  point  of  the  reactance  coil.  At  the  same  time,  the  solenoid  switch 
opens  the  circuit  through  the  shaking  coil,  through  the  starting  load 
resistance  and  through  the  starting  anode  resistance. 

When  the  wire  chief  wishes  to  stop  the  charge,  he  calls  29,  which  closes 
the  circuit  that  operates  the  310-ohm  relay  and  unlocks  the  switch  lever, 
whereupon  a  spring  draws  it  up,  opening  the  switch.  The  leads  to  the 
automatic  switchboard  are  taken  off  from  the  positive  and  negative  bus 


POWER  PLANT,  SUPERVISORY  AND  TESTING  EQUIPMENT     357 

bars,  shown  at  the  right-hand  end  of  the  diagram.  Each  negative 
lead  is  fused  as  indicated. 

Figure  286  shows  the  circuits  of  a  sub-office  power  plant,  arranged  so 
that  a  mercury  arc  rectifier  installed  at  the  sub-office  will  be  automatically 
started  up  and  commence  charging  the  battery  whenever  the  battery 
voltage  falls  to  46,  and  the  charge  will  be  stopped  whenever  the  battery 
voltage  reaches  60.  The  circuit  of  the  rectifier  is  practically  the  same  as 
that  shown  in  Fig.  285.  The  starting  and  stopping  relay  are  actuated 
by  a  Weston  high  and  low-voltage  alarm  relay.  When  the  voltage 
drops  to  46,  the  30-ohm  starting  relay  energizes  the  solenoid  controlled 
automatic  switch  which  connects  the  110-volt  alternating-current  power 
mains  with  the  leads  to  the  rectifier  transformer.  When  the  voltage 
reaches  60,  the  30-ohm  stop  relay  operates  the  unlocking  coil  of  the  auto- 
matic switch,  whereupon  the  switch  flies  open  and  stops  the  charge. 

Three  counter-e.m.f.  cells  are  provided  and  connected  to  a  switch 
shown  at  the  right  of  the  charging  control  switch,  so  that  while  charging 
is  going  on,  the  counter-e.m.f.  cells  are  switched  into  the  battery  supply 
circuit  of  the  harmonic  converter.  A  rheostat  is  provided  in  the  charging 
circuit  to  regulate  the  current  when  desired. 

In  the  lower  left-hand  corner  of  this  diagram,  circuits  are  shown  of 
apparatus  controlled  by  a  hair  hygrometer  for  regulating  the  humidity 
of  this  sub-office  which  is  one  of  a  number  installed  in  the  southern  portion 
of  the  United  States  where  no  heating  facilities  are  required,  except  those 
necessary  to  keep  the  relative  humidity  of  the  station  below  70  per  cent. 
When  the  needle  reaches  the  70  per  cent  mark,  it  energizes  the  500-ohm 
slow  relay.  This  relay  controls  the  automatic  switch  which  in  turn  closes 
circuit  through  an  electric  heater  and  a  12-inch  electric  fan.  This  com- 
bination circulates  heated  air  which  absorbs  the  surplus  moisture  through 
the  room.  When  the  humidity  has  been  sufficiently  reduced,  the  needle 
of  the  hygrometer  retires  below  the  70  per  cent  mark,  and  as  it  does  so,  it 
breaks  the  circuit  through  the  slow  relay.  Since  the  needle  vibrates 
somewhat  as  it  retires  from  the  contact  at  the  70  deg.  mark  and  does 
not  immediately  make  a  clean  break,  the  slow  relay  is  used,  as  it  will 
not  release  its  armature  until  the  hygrometer  needle  permanently  breaks 
the  circuit. 

Fuse  and  Voltage  Alarm  Scheme. — In  Fig.  287  is  shown  a  typical  fuse 
and  voltage  alarm  scheme.  Every  fuse  gives  an  indication  if  it  blows. 
Three  means  are  employed:  A,  the  alarm  fuse  with  an  alarm  strip;  B,  the 
cartridge  fuse  with  lamp  in  parallel ;  C,  cartridge  fuses  with  a  relay  con- 
nected across  two  of  them.  Practically  all  of  the  fuses  control  one  fuse 
alarm  bell,  by  means  of  a  low  resistance  relay  in  series  with  a  number  of 
red  tell-tale  lamps  on  the  ceiling. 

The  main  central  office  battery  on  the  power  board  is  protected  by 
enclosed  fuses.     These  fuses  have  alarm  relays  across  their  terminals, 


358 


.4  U  TOM  A  TIC  TELEPHON  Y 


two  windings  taking  care  of  four  fuses.     Each  relay  locks  mechanically 
and  lights  the  red  tell-tale  lamp  marked  "Power  Board." 

The  first  selector  boards  are  protected  by  alarm  fuses  which  connect 
negative  battery  on  to  an  alarm  strip.  This  operates  a  1300-ohm  relay 
which  in  turn  lights  a  fuse  pilot  lamp  in  series  with  a  low  resistance 


relay  common  to  the  first  selector  boards.     This  relay  lights  the  tell-tale 
lamp  on  the  ceiling. 

The  lineswitch  boards  are  similarly  protected,  except  that  the  fuse 
alarm  strip  is  connected  directly  to  the  fuse  pilot  lamp. 

The  circuit  breakers  are  provided  with  one  .1300-ohm  relay  to  indi- 
cate operation.  It  is  in  series  with  a  resistance  across  the  charging  lead 
but  short-circuited  by  the  circuit  breaker  when  closed. 


POWER  PLANT,  SUPERVISORY  AND  TESTING  EQUIPMENT     359 

The  supervisory  panel  which  contains  supervisory  apparatus  is  itself 
protected  by  fuses  of  the  alarm  type.  The  alarm  strip  connects  directly 
to  the  tell-tale  lamp. 

The  ringing-current  generator  fuses  are  of  the  alarm  type.  The  alarm 
strip  is  carried  to  a  1300-ohm  relay  which  puts  negative  battery  on  to  the 
tell-tale  lamp. 

A  failure  of  the  ringing  current  is  indicated  by  a  tell-tale  lamp  con- 
trolled by  a  4200-ohm  A.C.  relay  which  is  held  energized  as  long  as  the 
generator  supplies  current. 

The  ringing-generator  fuses  and  voltage  tell-tales  have  an  alarm  of 
their  own,  which  consists  of  a  bell  operated  by  dry  cells. 

The  main-battery  voltage  indicates  by  a  white  tell-tale  if  it  is  too  high 
or  too  low.  The  high-low  voltage  relay  on  the  bus  bars  controls  a  5000- 
ohm  slow  relay,  which  in  turn  lights  the  tell-tale  and  sounds  the  generator 
alarm  bell.  The  500-ohm  relay  has  a  copper  collar  to  make  it  slow  to 
respond,  so  as  not  to  indicate  momentary  extremes  of  voltage.  The 
arrangement  of  resistances  (2000  ohms  and  400  ohms)  is  to  protect  the 
contacts  of  the  voltage  relay. 

The  M.D.F.  heat  coils  light  a  tell-tale  lamp  by  means  of  a  common  low- 
resistance  relay,  but  do  not  sound  the  fuse  alarm  bell. 

Solenoid  Ringing  Interrupter. — In  Fig.  288  is  shown  the  circuit  of  a 
solenoid  ringing  interrupter.  When  the  "interrupter  start"  common, 
is  grounded  an  interrupter-start  relay  (Fig.  284),  will  energize.  The 
springs  on  the  interrupter-start  relay  are  multipled  so  that  whenever 
a  relay  becomes  operated  it  will  close  a  circuit  which  may  be  traced 
from  ground,  make  springs  on  the  solenoid  interupter,  the  two  windings  of 
the  solenoid  interrupter  in  series,  make  springs  interrupter-start  relay  to 
negative  battery.  The  solenoid  will  cause  the  plunger  of  the  solenoid 
interrupter  to  rise.  The  mechanical  arms  and  spring,  shown  midway  of 
the  plunger,  are  so  proportioned  that  the  springs  will  not  be  operated 
until  the  plunger  has  reached  the  highest  point  of  its  travel.  When  this 
point  is  reached  the  springs  will  disengage  and  open  the  circuit  of  the 
solenoid. 

As  the  plunger  with  its  rod  drops  by  force  of  gravity,  retarded  in  its 
movement  by  means  of  the  dash  pot,  the  double  wiper  furnishes  earth 
connection  successively  to  each  of  the  five  contacts  on  the  left-hand 
side.  As  the  wiper  is  passing  over  each  of  these  contacts  the  respective 
generator-control  relay  will  be  energized. 

When  the  plunger  reaches  its  lowest  point  of  travel  the  arms  again 
operate  and  as  a  result  ground  is  removed  from  the  back  of  contacts. 
If  the  interrupter-start  relay  is  still  energized  the  solenoid  interrupter 
will  operate  as  before.  The  cycle  of  operation  requires  about  three 
seconds  time — one-half  second  for  ringing  and  two  and  one-half  seconds 
interval  between  rings. 


360 


A  UTOMA  TIC  TELEPHON  Y 


Release  Signal  and  Tell-tale  Alarm  Scheme. — The  release  magnets 
of  all  switches  draw  a  heavy  current  momentarily;  the  same  is  true  of 
lineswitches  and  master  switches.  To  give  an  indication,  should  the 
current  flow  last  too  long,  a  system  of  release-signal  supervision  and 
ground-battery  supervision  is  employed.  It  involves  a  dash-pot  relay 
which  is  slow  to  act  but  relatively  quick  to  release. 

Referring  to  Fig.  289  it  will  be  found  that  the  selector  boards  have 
some  common  apparatus  in  the  first  board,  the  rest  of  the  boards  having 


Wiring/ 
of  Left -> 
Bank 


Solenoid 
Ringlnt  No.  1 

Fig.  288. — Solenoid  ringing  interrupter  circuit. 

only  the  pilot  lamp  and  the  shelf  leads.  Two  leads  are  carried  along  to 
all  the  boards,  the  tell-tale  wire  and  the  wire  for  the  time-limit  apparatus 
on  the  first  board  only. 

When  a  selector  releases,  its  release  magnet  draws  current  through  a 
0.6-ohm  relay  on  its  shelf.  This  sends  current  through  the  shelf  lamp 
which  is  in  series  with  a  15-ohm  relay  common  to  the  board,  which  in 
turn  draws  current  through  the  1300-ohm  relay  of  the  common  time-limit 
apparatus;  only  this  last  relay  energizes.  It  closes  the  circuit  of  the 
dash-pot  relay.     Ordinary  release  action  is  so  brief  that  the  dash-pot 


POWER  PLANT,  SUPERVISORY  AND  TESTING  EQUIPMENT     361 


2nd.  board 


First  Selector  Boards 
1st  Board 


Tell  Tale 

Lamps 

(On  Ceiling) 

lst.Se/. 


Other       Ep~ 
Boards     P9 


\Pito+ 


EtTtT 


I5l 


W 


©g/fcse 


Shelf    @ 

l  n TT A 


To  Rise  Magnet 


X+ 


2ncf.SheIf 

,4. 

To  Rise  Magnet 


2nd. Line  Swboard     1st  Line  Swboard 


1300 

rrmrr. 


I5W 

i  uuuy 


Dashfbt 
Relay 


280" 


h 


,U< 


2nd.Se/. 


Sameas 
1st.  Set. 

3rd.Se/. 


□ 


Ist.Group  Line 
Switchboards 


g1    Ib 


5/^ 
/%7 


toShelves  Grd.     5/g. 


Secondary  Line  Switchboard 
2nd3oard  1st.  Board 


Other 
boards 


\5upy0rd. 

[b 


B 


in  Any 


/2/5C 


5"> 

To 


Supy     M.S. 
Grd.      5i0. 


ran 


0     U<i  Dash 

Pot 

Relay 


2^jM 


man 

3/iow  rU 


Buzzer 


Key   ( 


/ 


IF 


□ 


Fig.  289. — Release  signal  supervision,  tell-tale  and  alarm  scheme. 


362  AUTOMATIC  TELEPHONY 

relay  has  not  time  to  act.  If  for  any  cause  the  release  magnet  of  a  selec- 
tor draws  current  longer  than  it  should,  the  dash-pot  relay  has  time  to 
close  its  contacts.  This  lights  the  tell-tale  lamp  and  shunts  the  1300-ohm 
relay  with  a  15-ohm  relay.  This  permits  the  15-ohm  relay  on  the  board 
to  pull  up,  lighting  the  pilot  lamp.  The  tell-tale  lamp  current  operates 
the  0.3-ohm  alarm  relay  which  causes  the  buzzer  to  sound.  The  second 
and  third  selectors  are  grouped  in  a  similar  way. 

The  lineswitch  boards  are  also  grouped  for  this  purpose.  The  super- 
visory apparatus  for  a  group  is  placed  on  the  supervisory  panel  of  the 
power  board  and  a  single  wire  run  to  the  group  of  boards.  There  is  one 
release  pilot  lamp  on  a  board  and  no  shelf  pilot,  for  all  the  connectors 
are  treated  as  if  one  shelf. 

The  lineswitches  and  master  switches  have  the  positive  or  ground 
battery  wire  "supervised."  This  means  that  it  passes  through  a  low- 
resistance  relay.  If  the  use  of  current  is  brief  no  signal  is  given ;  however, 
if  the  current  flows  too  long  the  time-limit  apparatus  will  light  the  tell- 
tale lamp,  the  pilot  lamp  and  also  sound  the  alarm  buzzer. 

Master  switches  have  also  a  "master  switch  signal"  which  indicates 
if  the  switch  sticks  between  positions.  The  lock  lever  will  then  put 
negative  battery  on  to  a  wire  leading  to  the  1000-ohm  winding  of  the 
supervisory  relay  for  the  board.  This  produces  the  same  results  as  if 
supervised  ground  were  acting. 

Secondary  lineswitch  boards  have  no  connectors,  hence  they  have  no 
release  signal.     Otherwise  their  supervision  is  the  same. 

Selector  and  Connector  Supervisory  Schemes. — The  action  of  selectors 
and  connectors  is  supervised  to  reveal  any  abnormal  action.  Normally 
a  selector  should  complete  its  selection  and  hunting  within  a  few  seconds. 
A  connector  should,  at  the  end  of  the  conversation,  release  promptly. 
Both  subscribers  having  hung  up  their  receivers  within  a  few  seconds  of 
each  other;  such  is  the  basis  of  supervision. 

There  is  a  white  tell-tale  lamp  on  the  ceiling  which  indicates  delayed 
action  for  all  selectors.  (See  Fig.  290.)  A  4-ohm  relay  acts  as  a  common 
for  all  the  group  of  selectors. 

The  release  relay  is  one  part  of  the  selector  whose  condition  is  indica- 
tive of  normal  progress.  Current  flows  in  this  coil  from  the  seizure  of 
the  selector  to  the  extension  of  the  lines  to  the  next  switch  ahead  and  the 
passing  of  the  control  from  its  selector.  Hence,  the  negative  current 
for  the  release  relay  is  drawn  through  a  14-ohm  relay  which  is  common 
to  the  shelf.  This  relay,  when  energized,  lights  a  pilot  lamp  for  the 
shelf.  The  shelf  pilot  lamps  draw  current  through  a  30-ohm  relay 
which  lights  the  pilot  lamp  for  the  bay. 

When  a  selector  is  seized  the  lamps  light.  When  the  switch  has 
selected  its  level  and  hunted  and  seized  the  idle  trunk,  the  lamps  go  out. 
Because  of  the  time  overlap  of  different  switches,  no  time-limit  apparatus 


POWER  PLANT,  SUPERVISORY  AN />  TESTING  EQUIPMENT    363 

can  be  used  and  no  alarm  signal  is  advisable.     Even  the  tell-tale  lamp  is 
capable  of  being  cut  off. 

The  trunk  tone  is  shown  in  connection  with  the  first  selectors;  the 
other  selectors  do  not  have  it. 


To  Other 
Boards 


Line  SwitchboardsOst. Group) 
3rd  Board  Pnd.  Board  Is  t  Board 


Supy  Tell 
7o/le  Lamps 
(OnCeilmgi 


%@ 


%Supy 

Pilot 
SupyV    \Supy 
#/       -i      4-  #^ 
Line  Switchboards  f  2nd  Group)to  Connector  Jacks 
3rd  Board  Prid  Board  1st  Board 


ToOfher 
Boards 


Supy 
Pilot 


Supy 
Pilot 


#2  tH|   "   |*2  #1 

First  Selector  Boards  (1st.  Board) 


Toother 
Boards 


h 


Low  Bat, 


m 


+J—J  rrmh 


n 


Same  as 
High  Bay 


+L.R 
To  Jacks  ot 
ZOSels 


Ind. 
Coil 


Rise  Ply 


To  Jacks 
ofPOSels 


High  Bay 


©Pilot 
w 


£J 


/TJTOI5-, 


'#*«-=- 


Tnd. 

Coil 


WS*-i 


To  0  ther  Shelves  To  Other  Shelves 

Second  Selector  Boards  (I  st.  Board) 
High  Bay 


Low  Bay 


+J- — Hif///;i 


Same  as 
High  day 


To  Jacks  of 
20  Sets  ■,  15 


^Supu 
^©Pilot 


To  Jacks  of    -t&~: 
POSels^       l5wQ  I 

Plse.Ply 


To  Other  Shelves 


To  Other  Shelves 

ZrdSel 


Like  2nd Sel. 
Fig.  290. — Selector  and  connector  supervisory  scheme. 


The  action  of  connectors  on  the  lineswitch  boards  is  supervised  by 
the  line  relay  and  the  back-bridge  relay.  As  long  as  both  subscribers 
are  on  the  line,  no  supervisory  lamp  need  show.     If  the  calling  subscriber 


364 


A  U  TO  MA  TIC  TELEPHOS  Y 


alone  hangs  up,  a  pilot  and  a  tell-tale  Lamp  will  show  tho  condition.     If 
only  the  called  subscriber  hangs  up,  the  same  lamps  will  light. 


TESTING  EQUIPMENT 

Standard  Impulse  Length  and  Speed. — One  of  the  essentials  of  the 
proper  operation  of  automatic  switchboard  telephone  equipment  is  the 
adjustment  of  the  central  office  switches  to  a  standard  calling-device 
impulse,  and  then  the  corresponding  adjustment  of  all  calling  devices 
so  that  they  will  furnish  this  standard  impulse  to  the  switches.  In  the 
end,  the  vital  point  is  the  performance  of  the  vertical  and  rotaiy  magnets 
to  which  impulses  must  be  delivered,  which  are  within  certain  limits  as  to 

frequency  and  character. 

To  accomplish  this  a  calling  device  is 
used  in  modern  two-wire  systems  which 
can  be  adjusted,  so  that  no  change  in 
impulse  adjustment  will  be  required  after 
it  is  installed  at  a  subscriber's  station, 
provided  the  central  office  switches  have 
been  adjusted  to  a  corresponding  standard 
impulse.  A  calling  device  of  the  type  re- 
ferred to  has  a  fiber  cam,  which  passes 
between  a  pair  of  floating  contact  springs. 
The  only  points  to  be  observed  in  the 
adjustment  of  the  cam  and  springs  are 
as  follows : 

First. — All  impulse  springs  should  be 
adjusted  so  that  the  ratio  between  the 
open  period  and  the  closed  period  will 
be  the  same  and  according  to  standard. 
Second. — The  speed  at  which  each 
calling  device  runs  should  either  be 
adjusted  to  suit  an  experienced  eye  and 
ear,  or,  better,  be  standardized  by  using 
a  speed  indicator. 

Calling  Device  Speed  Indicator. — This  little  piece  of  apparatus  (see 
Fig.  291)  is  used  to  compare  the  speed  of  a  calling  device  connected  to  it 
with  a  standard.  The  essential  parts  of  the  indicator  are  two  indicat- 
ing fingers,  each  of  which  is  fastened  to  a  small  wheel,  arranged  so  that 
it  may  be  rotated  from  left  to  right  through  an  arc  of  approximately 
90  deg. 

The  power  for  rotating  each  of  the  finger  wheels  consists  of  a  clock 
spring.  The  escapement,  which  allows  the  standard  finger  to  move  step 
by  step,  is  controlled  by  a  small  weighted  pawl  adjusted  to  ten  steps  per 


Fig 


291.— Calling    device 
indicator. 


speed 


POWER  PLANT,  SUPERVISORY  AND  TESTING  EQUIPMENT     365 

second.  The  escapement  of  the  other  ratchet  wheel  is  controlled  by  an 
electromagnet,  linked  with  the  line  of  the  calling  device  to  be  tested. 
The  ratchet  wheel  and  finger  escapes  one  step  for  each  calling-device 
impulse.  If  the  speed  of  the  calling  device  is  slower  than  the  standard, 
its  pointer  will  lag  behind  the  standard  pointer.  A  speed  indicator  of  the 
tuned  reed  variety  is  used  in  the  Siemens-Halske  exchanges,  instead  of  the 
type  just  described. 

Switch  Adjustment  and  Testing. — The  adjustment  of  an  automatic 
switch  of  the  Strowger  type  may  be  divided  into  two  parts,  the  relay 
adjustment  and  the  adjustment  of  the  motor  magnets. 

Each  relay  is  adjusted  to  a  definite  armature  stroke  with  a  fixed 
residual  air  gap.  The  amount  of  contact  of  each  spring  with  its  mate  is 
likewise  fixed  by  the  position  of  the  armature  when  the  contact  takes 
place.  These  distances  are  measured  with  thickness  gages  placed  between 
the  armature  and  the  pole  of  the  relay. 

The  spring  pressure  of  the  contact  springs  of  a  relay  is  measured  by 
the  operating  and  non-operating  currents.  For  instance,  with  the 
battery  voltage  as  closely  kept  between  46  and  48  as  possible,  a  certain 
relay  must  pull  up  through  3000  ohms  but  must  not  pull  up  through 
3200  ohms.  Such  a  pair  of  limits  is  set  for  each  relay  in  a  switch.  In 
the  case  of  quick-acting  relays,  it  is  assumed  that  in  the  factory  they  have 
passed  a  test  for  short-circuited  turns.  It  is  known  that  short-circuited 
turns  make  a  relay  slower  to  pull  up  and  slower  to  fall  back,  the  latter 
effect  being  more  noticeable. 

The  motor  magnets  (vertical  magnet,  rotary  magnet,  and  release 
magnet)  are  adjusted  in  a  definite  sequence  to  their  proper  relations  to 
the  wiper  shaft.  These  relations  are  expressed  in  distances,  most  of 
them  measured  by  thickness  gages. 

The  final  performance  of  a  switch  is  tested  by  being  operated  under 
line  conditions  which  are  worse  than  any  imposed  by  commercial  use. 
The  average  calling  device  on  a  telephone  delivers  a  61  per  cent  impulse 
at  about  ten  per  second.  (Circuit  opened  0.61  of  total  impulse  period.) 
The  testing  calling  device  or  "varying  machine"  delivers  impulses  at 
14  per  second  with  the  61  per  cent  ratio  under  two-line  conditions,  zero 
loop  with  a  20,000-ohm  leak,  and  1200-ohm  loop  without  leakage.  The 
spring  tension  of  the  magnet  (vertical  or  rotary)  is  adjusted  until  perfect 
operation  is  secured  under  these  two  extremes. 

SWITCH  TESTING  MACHINES 

Standard  impulse  machines  used  in  adjusting  the  central  office 
switches  are  furnished  in  portable  form  with  carrying  case,  in  which  are 
mounted  all  necessary  resistances  and  capacities  and  means  for  making  all 
changes  in  conditions  required  to  "vary"  any  switch  so  that  it  will  be 


366 


AUTOMATIC  TELEPHONY 


tested  under  variations  of  resistance,  capacity  and  speed  greater  than 
those  ever  experienced  in  receiving  impulses  from  a  subscriber's 
station. 

A  very  convenient  and  commonly  used  type  of  machine  consists  of  a 
small  (0.1  h.p.)  direct-current  motor  designed  to  run  at  a  speed  of  ap- 
proximately 400  revolutions  per  minute.  On  the  shaft  of  this  machine 
is  mounted  a  large  fiber  cam,  similar  to  that  used  on  a  telephone  calling 
device,  arranged  to  break  contact  twice  at  every  revolution  of  the  shaft, 
between  a  pair  of  floating  contact  springs,  mounted  on  a  bracket  secured 
to  the  base  of  the  machine.     It  is  readily  apparant  that  this  cam  and 


Fig.   292.- — A  view  showing  the  wire  chief's  desk  in  an  automatic  central  office. 

pair  of  springs  may  be  adjusted  to  give  an  impulse  of  the  same  character 
as  that  given  by  the  calling  device. 

The  set  has  flexible  cords  ending  in  a  clip  and  terminals.  The  clip  is 
attached  to  the  negative  battery  pole.  The  terminals  are  inserted  in  the 
test  jack  of  the  switch  to  be  tested.  The  motor  at  once  runs  so  as  to  give 
about  13  impulses  per  second  and  the  line  relay  of  the  switch  becomes 
energized.  Pressing  certain  buttons  causes  the  impulse  machine  to  give 
ten  impulses  followed  by  a  rest,  succeeded  by  another  set  of  ten,  indefi- 
nitely. One  button  operates  the  switch  through  zero  loop  resistance  with 
20,000  ohms  across  the  line,  the  other  through  1200  ohms  without  leakage. 
A  release  button  opens  the  line  to  release  the  switch. 

Wire  Chief's  Desks. — A  typical  wire  chief's  desk  (See  Fig.  292)  for  an 


POWER  PLANT,  SUPERVISORY  AND  TESTING  EQUIPMENT     367 

Automatic  Central  Office  contains  one  or  more  of  each  of  the  following 
circuits : 

Operator's  telephone  circuit 

Wire  chief's  test  circuit 

Trunks  to  test  jacks  on  main  frame 

Toll-test  plug  circuit 

Trunks  to  test  distributers 

In  and  Out  trunk  circuits 

Hospital  trunks 

Howler  circuit 

Out  order  wire  circuit 

In  order  wire  circuit 

Master  ringing  circuit 

Supervisory  pilot  circuit 

Fuse  alarm  circuit 

Lamp  circuits  for  supervisory 

Tell  tale  lamps  in  Main  exchange 

Sometimes  one  or  more  of  the  circuits  are  omitted,  sometimes  others 
are  added,  but  as  a  rule  a  list  similar  to  the  foregoing  is  used.  Testing  is 
usually  done  through  test  connectors  (one  per  100  lines)  reached  through 
a  test-distributer  switch. 

Description  of  Circuits. — In  Fig.  293  is  shown  a  typical  wire  chief's 
test  circuit  together  with  a  test-distributer  trunk. 

The  test  circuit  consists  of  a  high-resistance  voltmeter  and  sets  of  keys 
by  means  of  which  the  test  circuit  may  be  connected  to  any  one  of  a 
number  of  test-distributer  trunks,  or  to  the  main  distributing  frame. 

Besides  a  reversing  and  a  ringing  key  there  are  keys  for  connecting 
the  line  to  the  voltmeter  for  the  usual  line  tests  A . 

The  dial-speed  indicator  key  is  used  for  connecting  a  line  to  the  dial- 
speed  indicator,  for  the  purpose  of  receiving  impulses  from  a  telephone 
and  line  to  determine  whether  the  dial  is  operating  at  the  proper  speed 
or  not.  The  wire  chief  may  also  supply  current  to  the  telephone  on  the 
line  for  talking  purposes. 

At  the  right  of  the  figure  the  various  key  arrangements  for  the  differ- 
ent meter  tests,  which  may  be  made,  are  indicated. 

Trunks  to  Test  Distributers  and  Test  Connectors. — These  trunks 
enable  the  wire  chief  to  test  lines  without  assistance  and  without  leaving 
his  desk.  The  test  distributers  are  useful  in  other  ways,  however,  es- 
pecially where  district  stations  are  used.  A  test-distributer  switch 
installed  at  a  district  station  may  start  or  stop  the  charging  of  the  district 
station  storage  battery. 

These  switches  are  also  used  for  supervising  alarm  circuits,  etc.  The 
test-distributer  trunk,  test-distributer  switch  and  test-connector  switch 
circuits,  shown  in  Figs.  293,  294  and  295,  are  typical  of  those  in  com- 
mon use  in  two-wire  plants  using  equipment  of  the  Automatic  Electric 
Company's  manufacture. 


368 


A  UTOMA TIC  TELEPHON  Y 


POWER  PLANT,  SUPERVISORY  AND  TESTING  EQUIPMENT    369 

The  wire  chief  prepares  to  use  a  certain  test  distributer  by  operating 
the  associated  "test-distributer  key;"  he  then  operates  the  test-distrib- 
uter switch  by  making  two  turns  of  the  calling-do  vice  dial  associated 
with  his  test  circuits,  thus  placing  the  test  distributer  in  connection  with 
the  test  connector  mounted  on  the  particular  lineswitch  unit  in  which  the 
line  which  he  wishes  to  test  terminates. 

The  test-distributer  circuit  differs  from  others  in  the  following  details: 
A  pair  of  positive  and  negative  test  wires  pass  through  contacts  controlled 
by  the  private  armature  to  a  special  pair  of  wipers  mounted  on  the  switch 
shaft.  The  private  wiper  and  bank  contact  rows  are  double  like  the  line 
wiper  and  banks.  The  wiper  marked  P2  takes  the  place  of  the  ordinary 
private  wiper,  while  that  marked  PI  is  for  the  special  use  of  the  wire  chief 
in  supervisory  work. 

We  will  now  assume  that  the  tester  operates  the  test-distributer  key 
K  and  the  calling-device  keyL.  When  key  K  is  operated  this  particular 
test-distributer  trunk  is  cut  on  to  the  testing  circuit,  and  at  the  same  time 
a  circuit  is  closed  through  the  high  winding  of  relay  T.  When  key  L  is 
operated  a  circuit  is  closed  from  ground,  relay  C  (Fig.  294)  break-springs 
relay  F,  make  springs  key  K,  break  springs  B.C.O.  key,  make  springs 
key  L,  calling  device,  make  springs  key  L,  break-springs  release  connector 
key,  low-winding  relay  T,  non-inductive  resistance  A,  make  springs  key 
K,  break-springs  relay  F,  through  relay  A  to  negative  battery. 

Relay  T  will  not  operate  at  this  time  for  the  current  flowing  in  the 
lower  winding  will  oppose  the  current  in  the  upper  winding.  Relay  D  and 
A  will  operate  over  the  above-traced  circuit  and  place  the  test  distributer 
in  readiness  to  receive  the  dial  impulses. 

When  the  tester  dials  the  first  figure,  a  six  for  example,  the  circuit  of 
relays  A  an  D  will  be  opened  six  times.  Each  time  the  line  relay  A  de- 
energizes  a  circuit  may  be  traced  from  ground,  break-springs  relay  A, 
make  springs  relay  B,  series  relay  C,  vertical  magnets,  side-switch  wiper 
in  first  position,  to  negative  battery.  The  vertical  magnets  will  energize 
over  this  circuit  and  will  operate  to  raise  the  shaft  and  wipers  to  a  position 
opposite  the  sixth  level  of  bank  contacts.  The  series  relay  will  energize 
at  the  first  vertical  impulse  and  will  close  the  circuit  of  the  private 
magnet  P  and  relay  G;  the  energization  of  relay  G  has  no  effect  at  this 
time.  After  the  cessation  of  the  vertical  impulses  the  relay  C  will  de- 
energize  and  open  the  circuit  of  the  private  magnet;  the  private  magnet 
upon  de-energizing  will  allow  the  side-switch  wiper  to  pass  to  second 
position. 

When  the  tester  calls  the  next  figure,  a  seven  for  example,  the  relay  A 
will  momentarily  de-energize  seven  times,  and  will  send  seven  impulses  of 
current  over  a  circuit  which  may  be  traced  from  ground,  break-springs 
relay  A,  make  springs  relay  B,  series  relay  C,  break-springs  relay  E, 
rotary  magnets,  side-switch  wiper  in  second  position,  to  negative  battery. 


370 


AUTOMATIC  TELEPHONY 
is?1 


POWER  PLANT,  SUPERVISORY  AND  TESTING  EQUIPMENT     371 

The  rotary  magnets  will  energize  over  this  circuit  and  will  operate  to 
rotate  the  shaft  and  wipers  into  engagement  with  the  bank  contacts 
associated  with  test  connector  number  67.  The  series  relay  will  operate 
as  before  to  close  the  circuit  of  the  private  magnet.  After  cessation  of 
the  rotary  impulses  the  series  relay  will  de-energize  and  open  the  circuit 
of  the  private  magnet.  If  the  test  connector  in  board  67  is  idle  the 
private  magnet  P  will  now  de-energize  and  allow  the  side  switch  to  pass 
to  third  position. 

When  the  side  switch  passes  to  third  position  a  circuit  may  be  traced 
from  ground,  relay  H,  side-switch  wiper  in  third  position,  to  negative 
battery.  When  relay  H  energizes,  a  circuit  may  be  traced  from  ground, 
lower-winding  relay  A  (Fig.  295),  make  springs  relay  H,  make  springs 
relay  A,  upper-winding  relay  A,  to  negative  battery.  Relay  A  will 
energize  over  this  circuit  and  will  hold  the  test  connector  in  readiness  to 
receive  the  impulses  which  will  be  sent  it  by  the  line  relay  of  the  test 
distributer.  When  the  tester  dials  the  next  figure,  an  eight  for  example, 
the  line  relay  of  the  test  distributer  will  momentarily  de-energize  eight 
times,  each  time  a  circuit  will  be  closed  from  ground,  break-spring  relay 
A,  make  springs  relay  B,  series  relay  C,  make  springs  relay  H,  resistance 
K,  to  negative  battery.  Each  time  relay  A  drops  back  the  circuit  of 
relay  A  (Fig.  295)  is  opened.  Each  time  relay  A  (Fig.  295)  de-energizes 
a  circuit  may  be  traced  from  ground,  break-springs  relay  A,  make  springs 
relay  B,  off-normal  springs  (at  normal),  series  relay  D,  vertical  magnet, 
to  negative  battery.  The  vertical  magnet  will  energize. over  this  circuit 
and  will  operate  to  raise  the  shaft  and  wipers  to  a  position  opposite  the 
eighth  level  of  bank  contacts.  The  series  relay  D  will  energize  at  the  first 
vertical  impulse,  and  will  form  a  new  circuit  for  the  vertical  magnet  which 
which  may  be  traced  from  ground,  break-springs  relay  A,  make  springs 
relay  B,  off-normal  springs  (now  operated),  make  springs  relay  D,  relay 
D,  vertical  magnet,  to  negative  battery.  Following  the  cessation  of  the 
vertical  impulses  the  relay  D  will  de-energize  and  cannot  again  be  en- 
ergized because  its  circuit  is  now  open  at  the  break  contact  of  the  off- 
normal  springs. 

When  the  tester  calls  the  next  figure,  a  nine  for  example,  the  line  relay 
of  the  test  distributer  will  operate  as  before  to  repeat  the  impulses  to  the 
line  relay  of  the  test  connector.  Each  time  the  line  relay  A  (Fig.  295) 
de-energizes  a  circuit  may  be  traced  from  ground,  break-springs  relay  A, 
make  springs  relay  B,  off-normal  springs,  break-springs  relay  D,  relay 
C,  rotary  magnet,  to  negative  battery.  The  rotary  magnet  will  energize 
over  this  circuit  and  operate  to  rotate  the  shaft  and  wipers  into  engage- 
ment with  the  bank  contacts  associated  with  telephone  number  6789. 

It  will  be  seen  that  the  test  wipers  of  the  test  connector  are  held  open 
during  the  rotary  movement  by  the  springs  of  relay  C.  During  the 
rotary  motion  the  private  wiper  of  the  test  connector  is  held,  open  at  the 


372  AUTOMATIC  TELEPHONY 

springs  of  relay  C  (Fig.  294).  Shortly  after  the  last  figure  is  called  the 
relay  C  will  de-energize,  and  a  circuit  may  now  be  traced  from  ground 
break-springs  relay  G,  make  springs  relay  H,  break-springs  relay  C, 
make  springs  relay  D,  private  wiper  of  the  test  connector,  B.C.O.  winding 
of  the  called  line  switch,  to  negative  battery.  The  B.C.O.  winding  will 
energize  and  operate  to  clear  the  called  line  of  attachments.  We  now 
have  a  clear  circuit  from  the  called  line,  through  the  test  connector  and 
test  distributer,  to  the  test  circuit  on  the  wire  chief's  desk. 

The  tester  may  now  restore  the  calling  device  key  L  to  normal;  its 
springs  are  adjusted  to  "make  before  break,"  and  therefore  the  circuit 
will  still  be  held  up  through  relay  T. 

If  the  tester  operates  the  B.C.O.  key  a  shunt  will  be  placed  around 
relay  D.  (See  Fig.  294a.)  Relay  D  upon  de-energizing,  will  open  the 
circuit  of  the  B.C.O.  of  the  called  lineswitch,  thus  causing  the  line  relay  of 
the  called  lineswitch  to  be  bridged  across  the  line.  As  a  result  the  tester 
can  now  make  an  "in  test." 

If  the  tester  desires  to  release  only  the  test  connector  he  operates  the 
release-connector  key,  which  by  means  of  its  make  before  break  springs 
places  coil  F  in  the  circuit  in  place  of  relay  A  (Fig.  294a).  Relay  A, 
upon  de-energizing,  opens  the  circuit  of  relay  A  (Fig.  295b).  The  test 
connector  will  now  release  following  the  de-energization  of  its  line  relay. 
The  tester  can  now  operate  his  dial  to  call  any  number  to  which  this  test 
connector  has  access.  If  the  tester  so  desires  he  can  advance  the  test 
connector  across  the  level,  one  step  at  a  time,  by  simply  calling  the  figure 
one  on  his  dial. 

Let  us  assume  that  the  test  connector  in  the  67  board  is  busj^;  the  test 
distributer  will  now  pick  up  ground  on  the  associated  private-bank  con- 
tract and  as  the  series  relay  C  de-energizes  after  the  last  rotary  impulse  a 
circuit  may  be  traced  from  this  ground,  side-switch  wiper  in  second  posi- 
tion, upper-winding  relay  E,  break-springs  relay  C,  private  magnet  P,  to 
negative  battery.  The  private  magnet  will  remain  energized  over  this 
circuit,  and  will  therefore  hold  the  side-switch  wipers  in  scond  position. 
The  relay  E  will  energize  over  the  above  traced  circuit  and  place  the  busy 
tone  on  the  operating  line  to  indicate  to  the  tester  that  the  called  test 
connector  is  in  a  busy  condition.  The  busy  relay  also  opens  the  circuit 
to  the  rotary  magnet  so  that  a  further  rotation  of  the  switch  is  impossible. 
Let  us  assume  that  the  tester  leaves  the  connection  up  and  waits  for  the 
test  connector  to  become  idle.  As  soon  as  ground  is  removed  from  the 
private-bank  contact  the  private  magnet  P  and  the  bus}r  relay  E  will 
de-energize.  When  the  private  magnet  de-energizes  the  side  switch 
passes  to  third  position  and  relay  H  energizes  and  closes  the  operating 
circuit  to  the  test  connector.  In  order  to  allow  the  test  connector  to 
fully  release  before  its  operating  circuit  is  again  closed  the  private  magnet 
P  is.  made  slgw  acting.     The  de-energization  of  relay  E  caused  the  busy 


POWER  PLANT,  SUPERVISORY  AND  TESTING  EQUIPMENT     373 

tone  to  be  removed  from  the  line  and  the  tester  now  proceeds  to  complete 
the  connection. 

If  the  called  line  is  busy  a  circuit  may  be  traced  from  ground  at  the 
associated  private-bank  contact,  private  wiper  of  the  test  connector, 
private  wiper  of  the  test  distributor,  make  springs  relay  D,  break-springs 
relay  C,  make  springs  relay  H,  make  springs  relay  G,  relay  F  to  negative 
battery.  The  relay  F  will  energize  over  this  circuit  and  will  cause  the 
polarity  of  the  calling  line  to  be  reversed;  as  a  result  relay  T  (Fig.  293) 
will  now  operate  and  light  the  lamp  A  as  an  indication  to  the  tester  that 
the  called  line  is  busy.  The  relay  G  will  not  de-energize  following  the 
release  of  the  series  relay  C  but  will  be  locked  up  over  a  circuit  that  may 
be  traced  from  ground  at  the  busy  private-bank  contact,  make  springs 
relay  D,  break-springs  relay  C,  make  springs  relay  H,  make  springs  relay 
F,  make  springs  relay  G,  relay  G,  to  negative  battery.  When  the  called 
line  becomes  idle  the  protecting  ground  is  removed  and  as  a  result  relay 
F  and  relay  G  will  de-energize.  When  relay  F  de-energizes  the  polarity  of 
the  calling  line  will  be  returned  to  normal;  relay  T  will  now  de-energize 
and  extinguish  the  light  A  as  an  indication  to  the  tester  that  the  called 
line  has  become  idle.  When  relay  G  de-energizes  it  places  ground  on  the 
circuit  leading  to  the  B.C.O.  of  the  called  lineswitch.  Relay  G,  being  a 
slow-acting  relay,  will  allow  a  certain  interval  to  elapse  between  the 
removal  of  the  protecting  ground,  and  the  second  grounding  of  this  circuit. 
This  interval  is  required  in  order  that  the  called  lineswitch  may  have 
sufficient  time  to  remove  its  plunger  from  the  bank  before  its  B.C.O. 
winding  is  again  energized. 

If  the  tester  desires  to  leave  this  line  up  on  test  he  operates  the  hold 
key  and  restores  the  test-distributer  key  K.  The  hold  key  causes  a  short 
to  be  placed  across  the  operating  line  to  hold  up  the  connection,  and  at 
the  same  time  relay  H  is  bridged  across  the  line  under  test.  If  the  sub- 
scriber should  remove  his  receiver  he  would  draw  battery  through  relay 
H,  which  would  then  energize  and  light  lamp  B  as  an  indication  to  the 
tester  that  the  subscriber  had  come  in  on  the  line. 

After  the  test  is  completed  the  restoration  of  all  keys  will  cause  the 
circuit  of- line  relay  A  (Fig.  294)  to  be  opened.  When  relay  A  de-energizes 
it  opens  the  circuit  of  release  relay  B,  following  which  the  test  distributor 
releases  in  the  usual  manner.  The  de-energization  of  relay  A  also  opens 
the  circuit  of  line  relay  A  (Fig.  295),  following  which  the  release  of  the 
test  connector  takes  place. 

Information  and  Complaint  Desks. — Sometimes  a  desk  is  installed 
for  the  information  clerks,  separate  from  that  for  complaint  clerks,  in 
automatic  central  offices;  sometimes  both  are  placed  at  one  desk,  so 
arranged'  that  information  and  complaint  circuits  are  common  to  all 
positions  of  the  desk;  sometimes  in  small  offices  the  complaint  trunks 
terminate  in  the  wire  chief's  position,  or  in  a  second  position  in  the  wire 


374  AUTOMATIC  TELEPHONY 

chief's  desk.  In  small  offices  the  information  trunks  often  terminate  in 
a  position  on  the  long-distance  switchboard,  so  arranged  that  during  the 
day  it  is  presided  over  by  a  clerk,  especially  assigned  for  the  purpose, 
butat  night  and  on  holidays,  or  Sundays,  the  information  trunks  are 
attended  to  by  a  toll  operator. 

It  is  not  good  practice,  as  a  rule,  to  have  the  switchboard  attendant 
spend  time  answering  information  and  complaint  calls,  although  in  an 
office  up  to  1000  lines,  he  is  generally  able  to  attend  to  the  wire  chief's 
desks.  At  night  and  during  Sundays  and  holidays  one  employee  can 
generally  attend  to  the  automatic  switchboard,  the  wire  chief's  desk, 
and  the  combined  long-distance  information  and  complaint  desk,  where 
all  of  this  equipment  is  on  the  same  floor  and  in  adjacent  rooms.  If  it 
is  necessary  to  put  the  long-distance  switchboard  on  the  ground  floor  of  a 
central  office  building,  in  which  the  automatic  switchboards  are  on  the 
second  or  third  floor,  and  where  the  switchboard  is  for  1000  lines  or  less, 
it  may  be  advisable  to  have  the  complaint  trunks  terminate  in  one 
position  of  the  wire  chief's  desk;  and  if  the  office  is  quite  small,  that  is,  500 
lines  or  less,  they  may  be  attended  to  by  the  regular  switchboard  attend- 
ant; but  if  it  is  larger,  so  that  the  switchboard  attendant  does  not  have 
sufficient  time,  a  boy  or  a  woman  clerk  may  be  employed  to  receive  the 
complaints,  make  simple  tests  from  the  wire  chief's  position,  and  gen- 
erally assist  the  switchboard  attendant  during  the  busy  hours  of  the 
day. 

In  larger  offices  of  2000  lines  or  more,  the  information  and  complaint 
trunks  usually  terminate  in  a  desk  through  the  positions  of  which  both 
classes  of  trunks  are  multipled,  and  which  is  located  in  the  same  room  as 
the  long-distance  switchboard.  In  fact,  it  is  quite  desirable  to  use  the 
same  style  of  cabinet  for  the  information  and  complaint  desk  as  for  the 
long-distance  switchboard,  and  to  place  them  in  a  continuous  line,  arranged 
so  that  the  information  and  complaint  desk  may  grow  in  one  direction, 
while  the  long-distance  and  rural  positions  grow  in  the  other  direction. 
It  is  especially  desirable  to  have  them  in  the  same  room,  however,  so 
that  one  chief  operator  can  look  after  all  of  the  operators,  and  they  can 
all  use  the  same  retiring  room. 

In  multi-office  systems,  information  trunks  generally  terminate  at  one 
central  office,  the  business  headquarters  of  the  telephone  company. 
Sometimes  separate  complaint  desks  are  installed  at  each  of  the  main 
offices  in  such  a  system,  but  it  is  preferable  to  have  them  all  terminate 
in  the  same  office  as  the  information  trunks,  and  to  have  the  complaints 
distributed  to  the  wire  chiefs'  desks  in  the  various  main  offices  by  means 
of  telephones  or  telautographs. 

If  the  long-distance  switchboard  is  not  situated  in  the  building  with 
the  business  headquarters  of  the  telephone  company,  it  is  generally 
preferable  to  keep  the  information  and  complaint  desk  with  the  long- 


POWER  PLANT,  SUPERVISORY  AND  TESTING  EQUIPMENT     375 

distance  switchboard.     This  simplifies  the  supervision  and  arranging  of 
reliefs,  promotes  economy  in  the  provision  of  rest-room  facilities,  etc. 

For  all  systems,  the  following  circuits  generally  terminate  in  the  in- 
formation desk,  when  it  is  installed  separately  from  the  complaint  desk: 

Incoming  information  trunks. 

Incoming  dead-number  trunks. 

Where  part.y  lines  are  used  and  reverting  calls  are  handled  by  means  of 
an  operator,  who  rings  the  party  desired  on  a  reverting  call,  trunks  for 
this  work  also  usually  terminate  in  the  information  desk. 

When  the  complaint  desk  is  separate  from  the  information  desk, 
the  following  circuits  terminate  in  it: 

Incoming  complaint  trunks. 

Incoming  trunks  from  dead-bank  levels  or  contacts. 

The  following  circuits  are  used  in  desks  of  both  types: 

Trunks  between  desks. 

In-  and  out-trunk  circuits. 

Out-order  wire  circuits. 

In-order  wire  circuits. 

Outgoing  trunks  to  the  automatic  switchboard. 

Operators'  telephone  circuits. 

Position  switching  circuits,  for  desks  of  more  than  one  position. 

Pilot  and  night-alarm  circuits. 

Incoming  Information  Trunks. — Circuits  of  a  typical  incoming  infor- 
mation trunk  from  a  level  of  selector  banks  to  an  information  clerk's 
position  are  shown  in  Fig.  296.  In  this  diagram  detail  "  Fig.  1 "  shows  the 
circuit  connections  for  a  one-position  desk,  while  "Fig.  2"  shows  the  circuit 
connections  for  a  desk  of  two  or  more  positions,  through  which  the 
incoming  trunk  is  multipled.  It  is  customary  to  wire  these  trunks  to  a 
level  of  the  automatic  switchboard  selector  banks  in  such  a  manner  that  a 
subscriber  desiring  to  call  the  information  clerk  does  so  by  making 
one,  two,  or  at  the  most,  three  turns  of  his  dial.  This  is  determined 
by  whether  the  trunks  terminate  in  first,-  second-  or  third-selector  banks. 
In  small  one-office  systems,  they  usually  terminate  in  first-selector 
banks;  in  larger  systems,  in  second-selector  banks,  in  large  multi-office 
systems,  they  sometimes  terminate  in  third-selector  banks. 

Upon  calling  the  number  designated,  as  that  of  the  information  clerk, 
the  selector  switch  operated  by  the  subscriber,  in  accordance  with  the  last 
digit  of  the  number,  selects  an  idle  trunk  to  the  information  operator  and 
then  extends  the  subscriber's  line  through  to  it.  When  this  occurs,  cur- 
rent flows  from  battery  through  the  windings  of  the  double-wound 
250-ohm  line  relay,  shown  in  Fig.  296,  and  through  the  subscriber's  loop. 
When  this  relay  attracts  its  armature,  it  lights  the  line  signal  lamp  or 
lamps,  and  grounds  the  release  trunk. 

The  clerk  responds  by  throwing  a  key  associated  with  the  trunk  in 


370 


A  U  TOM  A  TIC  TELEPHON  Y 


her  position,  thus  connecting  her  transmitter  and  receiver  across  the 
trunk,  and  operating  the  1000-ohm  cut-off  relay.  When  this  relay 
operates,  it  cuts  off  the  signal  lamp  and  grounds  the  release  trunk  to 
prevent  release  by  the  subscriber.  It  closes  also  a  locking  circuit  from 
earth  through  the  springs  of  the  line  relay  and  its  own  winding  to  negative 
battery,  so  that  after  once  attracting  its  armature,  it  will  not  release  it 
until  both  the  subscriber  and  the  operator  are  off  the  circuit.  The 
cut-off  relay  lights  the  holding  lamps  shown  just  above  the  line-signal 
lamps,  so  that  as  long  as  the  subscriber  remains  on  the  trunk  these  hold- 
ing lamps  will  glow  to  prevent  the  call  from  being  forgotten. 

Use  only  when 

Specified 


l*.!==*rn     '  I  j,  When  Key  i 

„ o-i    LK   cut  Loop : 


When  Key  is  used 

--  X 


To  Oper 


Fig.  296. — Trunk  incoming  to  an  information  position  from  selector  switch  banks. 

Occasionally  information  boards  are  arranged  with  the  trunks  ending 
in  jacks  instead  of  keys.  The  circuits  of  a  jack-ending  trunk  are  shown 
by  the  dotted  lines  in  Fig.  296. 

Dead-number  Trunks. — These  trunks  are  used  to  connect  the  con- 
nector bank  contacts,  coresponding  to  numbers  which  appear  in  the 
directory,  but  which  are  not  in  use,  to  the  information  switchboard,  so 
that  a  subscriber  calling  such  a  number  will  be  automatically  switched 
to  an  information  clerk,  who  will  give  him  attention.  Generally  the 
connections  from  the  dead  lines  to  the  dead-number  trunks  are  made  at 
the  main  distributing  frame,  but  another  practice  is  to  make  them  at  the 
connector  bank  cable  terminals. 

Where  party  lines  are  used  the  latter  method  is  decidedly  prefer- 
able, because  it  will  not  be  desired  to  connect  an  entire  party  line  to  the 


POWER  PLANT,  SUPERVISORY  AND  TESTING  EQUIPMENT    377 

dead-number  trunk.  It  is,  therefore,  customary  to  arrange  the  con- 
nector-switch banks  on  party-line  boards,  with  the  banks  of  each  group 
of  connector  switches  connected  to  a  separate  terminal  strip.  This 
makes  it  possible  to  disconnect  the  bank  multiple  of  any  particular 
group  of  connector  switches  from  its  line,  and  to  connect  that  multiple 
to  a  dead-number  trunk. 

Another  method  of  handling  dead-number  trunk  connections  for 
party  lines,  but  one  which  is  considerably  more  complicated  and  expen- 
sive, is  to  substitute  for  the  telephone  removed  from  any  line  a  special 
relay  connected  to  the  line  at  the  central  office  and  designed  to  operate 
only  on  ringing  current  of  the  same  frequency  to  which  the  ringer  of  the 


Second  Sect. 


^ 


TO  Pl/.RIu. 


r-L 


CO 


First  Section 


(c 


"-1 


To  Oper. 


a 


To  Pi  J.  Relau 


•^  ,  —  -Z       1  I  '     To  Pi  I.  Relau 


ir^Hrti 


\When  Key  is  used  cut 
|  Loop  X 


FIG.  2 


B 

v  *    Z 

FIG.  1 

C 

sy*     tz 

Oper.  set  used            '  +       ° — 

must  hare  cond. 

in  line  CCT.                            ~= 

D 

To  Oper. 

% 


X£ 


All  privates  of 

dead  lines  conn. 

I  to  one  trk.  I.c."m." 

Mustbemultipled 
\  The  Priv.  nor.  line 
iSw.to  be  opened  at 
the  Jack. 


5.3 


y 


? 


A 


1000 


Fig.   297. — Trunk  for  connecting  dead  lines  or  numbers  to  an  information  position. 


removed  telephone  would  respond.  When  a  subscriber  calls  the  number 
corresponding  to  the  removed  telephone  this  relay  serves  to  connect 
the  line  temporarily  to  the  dead-number  trunk.  This  method  of  handling 
dead-number  trunks  must  be  used  on  party  lines  where  the  design  of  the 
switchboard  is  such  that  it  is  not  possible  to  isolate  the  connector-bank 
multiple  corresponding  to  the  dead-number. 

A  dead-number  trunk  for  use  where  the  disconnecting  of  connector- 
bank  multiples  is  the  scheme  followed  for  connecting  up  dead  numbers  is 
shown  in  Fig.  297.  In  this  figure  the  detail  "Fig.  1 "  shows  the  method 
of  connecting  to  a  key  in  a  one-position  desk,  while  detail  "Fig.  2" 
shows-  the  method  of  multiplying  the  trunk  through  keys  in  two  or  more 
positions. 


378 


A 11  TOM  A  TIC  TELEPHON  Y 


When  a  subscriber  connects  to  one  of  these  dead  trunks,  the  current 
flowing  through  the  back-bridge  relay  windings  of  the  connector  switch 
used  by  him  energizes  the  1000-ohm  relay  A.  This  relay  lights  the  signal 
lamp  through  conductor  A.  The  operator  responds  by  throwing  a  key 
in  her  position  which  bridges  her  transmitter  and  receiver  across  the  line 
and  energizes  the  cut-off  relay.  The  cut-off  relay  opens  the  circuit 
through  the  lamp  and  closes  a  locking  circuit  through  its  own  winding. 
It  also  opens  the  shunt  across  the  5000-ohm  resistance  coil,  thus  placing 
that  coil  in  series  with  the  coil  of  relay  A.  The  purpose  of  this  is  to  pre- 
vent sufficient  current  flowing  through  the  back-bridge  relay  windings 
of  the  connector  switch  to  operate  them.     If  the  party  has  called  from  a 


O 


v_£ 


-T 


£  R 


To  Pit.  Relay 


NEH 


Arranged  to  handle  calls 
on  pay  station  fi,  metered  tines 
wirh  out  deposit  of  coin  or 
registering  meter. 

Oper's  set  used  must  have 
cond  in  line  ckt. 


Re/ay  A  adjusted  to 
tiold  up  thru  6000 a>  and 
adjusted  not  to  pull  up  tvith 
Oen.  Current 


5000  cu<- 


I 


IF 


D 


i 
i 
i 
i 
i 


^ 


;j 


£X 


3100  uj 
A 


M.D.F 


I50uj    C 
.    500  lu^- 


u 


tC 


Fig.  298. — Dead  number  trunk  circuit  for  use  in  an  exchange  having  metered    lines   or 
sub-station   coin   collectors  actuated   by   "reversing  battery"   connector  switches. 


paystation  telephone,  equipped  with  a  coin  collector  using  a  polarized  re- 
lay, this  relay  will  not  be  operated,  and  the  subscriber  is  not  required 
to  deposit  a  coin. 

Another  type  of  dead-number  trunk  circuit  is  that  shown  in  Fig.  298, 
which  is  designed  especially  for  use  where  meters  are  employed,  which 
register  as  soon  as  the  current  in  a  calling  subscriber's  loop  reverses  at  the 
instant  the  called  party  responds.  This  circuit  is  provided  with  a  3100- 
ohm  slow-acting  relay  A  bridged  across  the  line  and  operated  by  the 
talking  current  of  the  connector  switch.  A  being  slow,  will  not  respond 
to  ringing  current.  A  closes  circuit  from  earth  through  its  own  springs, 
the  back  contact  of  the  1000-ohm  relay  and  the  signal  lamp  to  the  pilot 


POWER  PLANT,  SUPERVISORY  AND  TESTING  EQUIPMENT    379 

relay  and  negative  battery.  It  also  closes  circuit  by  means  of  the  same 
spring  contact  through  the  150-ohm  winding  of  the  induction  coil  B. 
The  induced  current  which  is  generated  for  an  instant,  as  the  circuit 
through  this  winding  of  B  is  closed,  in  the  500-ohm  winding  of  B  and  the 
500-ohm  winding  of  C,  causes  C  to  attract  its  armature  for  an  instant. 
This  short-circuits  the  line  long  enough  for  the  ring  cut-off  relay  of  the 
connector  switch  to  operate  and  switch  the  ringing  relay  out  of  circuit; 
but  it  does  not  cause  the  back-bridge  relay  of  the  connector  switch  to 
reverse  the  direction  of  current  flow  in  the  calling  party's  loop  for  a 
sufficient  period  of  time  to  operate  the  meter  associated  with  the  calling 
party's  line  switch.  It  is  therefore  seen  that  by  the  use  of  this  circuit  a 
subscriber,  whose  line  is  equipped  with  a  meter,  may  call  the  information 
clerk  on  a  dead-number  trunk  without  having  the  call  registered. 


To  out  O.WKeijs 


Fig.  299. — Operator's  circuit,  including  calling  device,  for  an  information  or  complaint 

desk. 


Incoming  Complaint  Trunks. — These  trunks  usually  terminate  at  one 
end  in  the  banks  of  selector  switches,  and  at  the  other  end  in  keys  in  the 
complaint  or  information  clerk's  position,  and  are  in  every  way  similar 
to  the  incoming  information  trunks  already  shown  in  Fig.  296  and 
described  in  the  preceding  paragraphs. 

Operator's  and  Calling  Device  Circuit.— A  typical  operator's  talking, 
listening,  and  calling  device  circuit  for  an  information  or  complaint 
switchboard,  is  shown  in  Fig.  299.  The  transmitter  of  this  circuit  is 
equipped  with  anti-side  tone  features;  leads  are  provided  to  out  order- 
wire  keys,  to  listening  keys  on  incoming  trunks,  and  to  calling-device 
keys.  A  calling  device  is  wired  into  the  circuit  in  such  a  way  that  con- 
nection leads  labeled— "To  CD.  Keys"— to  an  outgoing  trunk  will  close 


380 


AUTOMATIC  TELEPHONY 


the  calling  loop  through  circuit  leading  from  the  lead  marked  R,  through 
the  calling-device  impulse  springs  IS,  the  380-  and  80-ohm  windings  of  the 
induction  coil,  the  91-ohm  secondary  induction  coil  winding,  to  the  other 
side  T  of  the  line.  At  the  same  time  the  operator's  receiver  is  bridged 
across  the  trunk  by  a  circuit  leading  from  R  through  IS,  the  receiver, 
the  condenser  S,  the  secondary  winding  of  the  induction  coil,  and  the 
other  side  T  of  the  trunk.  Whenever  the  calling  device  dial  is  turned  it 
shunts  out  the  receiver  by  closing  together  the  springs  SS  while  impulses 
are  being  sent. 

Outgoing  Trunks   to   the   Automatic    Switchboard. — A   key   ending 
outgoing  trunk  to  a  lineswitch  on  the  automatic  switcnboard  for  use  by 


<s 


/T 


Vf 


/" 


r 


1 


■AAWVh 

500  u) 


M.D.  F. 


To  Line  Sw. 


To  opers.  Tel.  Set 

Fig.   300. — Outgoing  trunk  circuit  from  an  information  or  complaint  position  to  an  auto- 
matic switchboard. 


an  information  clerk  in  calling  subscribers,  by  means  of  the  calling 
device  installed  in  her  position,  is  shown  in  Fig.  300.  To  make  a  call 
the  clerk  throws  the  key,  which  switches  her  set  onto  the  outgoing 
trunk,  and  operates  the  calling  device  in  the  usual  manner.  If,  after 
securing  the  connection,  she  should  wish  to  switch  her  set  off  the  trunk 
without  releasing  the  connection,  she  throws  the  other  key,  shown  in  the 
diagram,  which  bridges  a  500-ohm  resistance  across  the  trunk  and  also 
closes  circuit  through  the  holding  signal  lamp  and  battery.  If,  when  she 
is  ready  to  leave  the  trunk,  she  does  not  wish  to  hold  the  connection  she 
simply  restores  her  talking  set  key  to  normal,  and  since  this  opens  the 
the  line,  the  automatic  switches  used  instantly  release. 


CHAPTER  XVI 
TRAFFIC 

A  knowledge  of  the  amount  of  traffic  in  a  telephone  exchange  and  its 
distribution  with  respect  to  time  and  destination,  is  essential,  to  be  able  to 
provide  central  office  equipment  which  is  adequate  for  the  subscriber's 
needs. 

In  the  planning  of  an  exchange  it  is  important  that  the  network  be  so 
designed  as  to  accommodate  the  anticipated  traffic  demands  on  an  eco- 
nomic basis,  since  in  no  other  industry  is  there  found  such  a  heavy  plant 
investment  for  the  relatively  short  period  of  its  useful  service. 

From  the  standpoint  of  the  subscriber,  telephone  service  does  not 
measure  up  to  its  fullest  efficiency  unless  facilities  are  available  for  com- 
pleting his  connection  whenever  desired.  On  the  other  hand,  to  provide 
the  necessary  equipment  to  take  care  of  all  emergencies  and  traffic 
demands  not  anticipated,  would  require  an  investment  that  would  entail 
such  high  charges  for  service  as  to  defeat  its  own  purpose.  For  good 
commercial  telephone  service,  therefore,  we  are  chiefly  interested  in 
knowing  how  much  switching  equipment  to  provide  so  that  not  more 
than  a  certain  number  of  calls,  say  an  average  of  1  to  100,  or  1  to  1000 
will  be  lost. 

Definitions. — The  word  traffic  as  used  in  telephone  practice  applies  to 
the  volume  of  calls  which  a  sj^stem  handles.  The  measure  of  traffic  is 
commonly  the  individual  call,  but  for  some  purposes  it  is  necessary  to  take 
the  average  duration  of  each  connection  or  call  into  consideration  also. 
A  knowledge  of  traffic  and  its  laws  is  essential  to  the  proper  design  or 
operation  of  either  an  automatic  or  a  manual  system.  In  a  discussion  of 
traffic  the  following  terms  are  used  and  their  meanings  should  therefore 
be  understood. 

Busy  Hour. — That  hour  of  the  day  during  which  the  greatest  number 
of  connections  are  made.  In  the  average  system,  the  busy-hour  traffic 
represents  about  %  (but  varies  from  %  to  }{ 2)  of  the  day's  total  business. 
(See  Fig.  301.) 

Peg  Count. — A  count  of  all  the  connections  made  during  a  definite 
time  by  each  of  the  operators  on  a  manual  switchboard. 

Plug  Count. — A  count  at  a  definite  instant  of  all  plugs  inserted  in  the 
jacks  of  a  manual  switchboard. 

Traffic  Measurements. — -In  automatic  central  offices,  counts  corre- 
sponding to  peg  counts  are  occasionally  taken  by  the  attendants  by  count- 
ing the  machines  as  they  complete  connections.     Calls  are  also  counted 

381 


382 


AUTOMATIC  TELEPHONY 


by  the  use  of  meters.  These  are  generally  installed  by  connecting  the 
release  magnets  of  second  and  third  selector  switches  on  each  section  of 
the  switchboard  to  a  separate  bus  bar,  in  series  with  which  is  a  J^-ohm 


50 
45 

40 

35 

^30 

z 
I- 
< 

|25 

a 

o 

°20 
a 

a 

I 

10 

b 

12M.    2a.m.     4A.M.   6A.M.    8A.M.  10a.m.   12m.    2P.M.   4 
TIME  OF  DAY 


p.m.   6p.m.   8p.m.  10  p.m.    12m. 


Fig.   301. — Typical  load  curve  of  a  central  office  for  a  24-hour  period. 

relay  which,  each  time  a  switch  releases,  operates  and  closes  a  circuit 
through  a  meter.  Counts  similar  to  plug  counts  have  been  taken  by 
counting  all  of  the  like  machines  in  operated  condition  at  given  times. 


.  Fig.  302. — Traffic  recording  machine. 

The  easiest  way  that  has  been  used  to  ascertain  the  maximum  traffic 
through  a  common  battery  manual  switchboard  is  to  insert  a  recording 
ammeter  in  the  leads  supplying  talking  and  operating  current.  By  taking 
the  peak  of  the  ammeter  curve  and  dividing  it  by  the  current  used  per 


TRAFFIC  383 

connection  an  accurate  idea  of  the  maximum  number  of  simultaneous 
connections  may  be  secured.  This  is  much  more  accurate  than  a  plug 
count  because  pairs  of  plugs  up  do  not  in  a  considerable  per  cent  of 
cases  represent  uncompleted  conversations,  especially  during  the  rush 
hours. 

In  the  proceedings  of  the  American  Institute  of  Electrical  Engineers 
Mr.  W.  Lee  Campbell  describes  the  traffic-recording  machine  (Fig.  302) 
which  he  used  to  obtain  much  traffic  data.  Forty  styluses  in  the  center 
make  records  on  a  broad  tape  which  passes  from  the  roll  at  the  right 
to  the  clock-driven  roll  at  the  left.  Each  stylus  is  connected  by  a  relay 
to  the  release  trunk  of  a  group  of  trunks  which  is  under  study.  As  long 
as  a  trunk  is  occupied,  the  stylus  for  it  draws  a  line  on  the  paper  tape. 
For  details,  the  reader  is  referred  to  the  paper  cited. 

Average  Traffic  Requirements. — Although  the  amount  of  apparatus 
placed  on  an  automatic  switchboard  is  the  subject  of  much  careful 
engineering,  a  general  idea  may  be  had  of  the  customary  numbers  of 
switches  placed  at  the  disposal  of  hundred-line  units. 

With  lineswitch  boards  equipped  with  Keith  lineswitches,  whose 
banks  have  ten  points,  from  eight  to  20  trunks  are  usually  provided. 
The  latter  is  secured  by  dividing  the  lineswitches  into  two  groups  of  50 
each.  In  some  extreme  cases  of  party-line  boards  it  has  been  necessary 
to  give  them  40  trunks,  by  dividing  the  lineswitches  into  four  groups  of 
25  each,  but  naturally  the  efficiency  of  the  trunks  is  low. 

Rotary  lineswitch  boards  have  25  trunks  in  a  group.  Individual 
line  boards  may  be  arranged  from  200  to  500  lines  per  group  of  25  trunks. 
Two-party  line  boards  will  run  from  300  to  400  lines  per  group.  Four- 
party  line  boards  may  have  200  to  300  lines  per  group  of  25  trunks. 

The  arrangement  of  selectors  other  than  firsts  is  calculated  according 
to  principles  which  will  be  explained  in  detail  later  in  this  chapter. 

Connectors  are  furnished  for  lineswitch  boards  approximately  accord- 
ing to  the  following  table: 


Average 
installed 


Ultimate 


Keith  ltotary 


Individual  lines 10  12 

Two-party  lines 10  to  12  18 

Four-party  lines 20  to  24    |         28 


1(1 
16 


In  each  case,  one  of  the  connectors  is  a  test  connector,  not  available 
for  public  traffic. 

EFFICIENCY  OF  TRUNK  GROUPS 

In  making  a  study  of  the  carrying  capacity  of  automatic  and  manual 
trunk  groups  of  various  sizes,  several  truths  become  apparent. 


384 


A  I   TOM  A  TI(  ■  TELEPIION  Y 


1.  A  largo  trunk  group  has  a  greater  carrying  capacity  per  trunk  than 
a  smaller  group. 

2.  Storage  of  calls  for  a  few  seconds  at  a  time  is  common  practice 
among  manual  operators  and  enables  them  slightly  to  smooth  out  the 
trunk  load  curve. 

Automatic  subscribers  attain  a  similar  storage,  because  if  a  user  finds 
that  he  can  not  get  through  to  his  destination,  he  hangs  up  and  dials 
again.  Very  often  the  second  time  he  finds  the  trunk  group  with  an 
opening. 

3.  An  automatic  trunk  group  will  carry  more  calls  than  a  manual 
group  of  the  same  size. 

The  first  truth  is  strikingly  illustrated  by  the  following  facts:  It  has 
already  been  stated  that  it  is  common  practice  in  automatic  systems  to 
provide  8  to  10  per  cent  of  trunking  for  outgoing  calls  from  a  group  of  100 
line  switches,  because  experience  has  shown  that  this  number  is  required. 
It  has  sometimes  been  necessary  to  install  ten  trunks  for  a  group  of  50 
lineswitches,  but  if  all  the  calls  being  made  in  a  large  office  could  be 
handled  through  one  group  of  trunks,  a  very  much  smaller  percentage 
would  be  required.  To  show  how  small  this  might  be,  counts  were 
made  of  all  the  connections  up  at  various  moments  during  the  busy  hour 
of  every  day  in  a  week  in  a  number  of  automatic  offices.  From  the  num- 
bers counted  at  each  office,  the  largest  was  selected  and  placed  in  the 
following  table: 


Office 

Number  of  lines 
in  service 

Number  of  tele- 
phones in  service 

Max.  no.  of  con- 
nections up  at 
one  time 

Ratio  of  max.  no. 
of  connections  up 
to  no.  of  lines  in 
service  in  per  cent 

A 

9,300 

10,300                      258 

2.7 

B 

7 ,  950 

9,914 

283                        3 . G 

C 

1,884 

2,091 

40                        2.1 

D 

1,870 

1,943 

33                        2.0 

E 

1,075 

1,150                         25                         2.3 

F                               969 

1 ,  008                        34                        3.5 

G                                921                     1,199                         42                         4.5 

The  average  call  rate  was  from  ten  to  twelve  per  line  per  day. 

This  list  indicates  that  if  the  trunks  could  be  brought  up  to  full 
efficiency  a  number  equal  to  not  more  than  3^  per  cent  of  subscribers 
lines  would  be  required  in  the  average  office. 

It  has  already  been  found  possible  to  reduce  the  per  cent  of  first 


TRAFFIC 


385 


selector  switches  to  5  in  offices  carrying  an  average  load  by  the  use  of 
secondary  lineswitches,  which  combine  100  trunks  into  one  group,  serving 
about  2000  lines. 

The  great  lack  of  efficiency  in  small  trunk  groups  is  due  to  the  erratic 
fluctuations  in  the  traffic  from  moment  to  moment.  It  is  not  unusual  for 
every  trunk  in  one  group  of  ten  to  be  busy  at  a  moment  when  in  a  neigh- 
boring group  all  or  nearly  all  trunks  are  idle,  and,  five  minutes  later,  to 
find  the  conditions  just  reversed. 

The  great  variations  in  the  traffic  of  small  trunk  groups  as  compared 
with  larger  groups  is  illustrated  by  the  curves  in  Fig.  303  in  which  curves 
A  show  the  ratio,  in  per  cent,  between  the  number  of  trunks  in  use  and 


A        ENTIRE  OFFICE  OF  7ECO  WORKING  UINES 

B       A  1000  SECTION  CONTAINING  060  WORKING  LINi 

C       A  100  GROUP  CONTAINING  95  WORKING  LINES 


8% 

7*. 
o 
o 

6S?£ 
5%%' 

£>   J 

z> ; 

Z  i 

3*1  J 

< 


MONDAY  TUESDAY  WEDNESDAY  THURSDAY  FRIDAY  SATURDAY 

Fig.  303. — Curves  showing  ratios  of  trunks  in  use  to  subscriber  lines  during  the  busy 

hour  of  each  day  of  a  week. 


the  subscribers'  lines  in  service  at  half-hour  intervals  during  the  busy 
period  of  each  working  day  of  a  week  in  the  office  designated  as  B  in  the 
preceding  list. 

The  curves  B  in  this  figure  show  the  ratios  at  the  same  time  for  a  1000 
section  carrying  an  average  load  and  the  curves  C  show  similar  ratios  for 
a  100-line  group  handling  a  normal  number  of  connections  in  this  office. 

It  will  be  noted  that  the  variations  in  A  are  comparatively  much 
smaller  than  those  in  B  while  C  varies  very  widely. 

The  second  truth  on  page  384  is  readily  understood.  If  two  calls 
come  to  a  manual  operator  at  the  same  time,  one  must  wait  until  she 
cares  for  the  other.  In  this  way  each  operator  slightly  smooths  out  the 
smaller  and  quicker  variations  in  the  load  curve.  On  an  automatic 
switchboard,  this  has  not  been  found  practicable.     Every  call  must  be, 

25 


380 


AUTOMATIC  TELEPHONY 


handled  without  a  delay  of  even  a  fraction  of  a  second  or  else  the  sub- 
scriber must  hang  up  and  dial  again. 

In  spite  of  this  fact,  the  third  truth  is  apparent  to  any  one  who  has 
made  a  study  of  the  matter,  viz.,  that  an  automatic  trunk  group  will 
carry  more  calls  than  a  manually-operated  group  of  the  same  size.  There- 
fore, if  an  automatic  system  is  to  displace  a  manual  system,  the  busy-hour 
peg  counts  and  plug  counts  of  the  operators  may  be  used  to  ascertain 
the  number  of  automatic  trunks  required  but  the  number  of  cords 
installed  or  found  in  use  at  the  busiest  moment  on  the  manual  board 
should  not  be  taken  as  the  number  of  automatic  trunks  necesary. 


150 

/ 

130 

'i 

120 

A 

V 

P 

<ft 

110 

o 

•o 

§100 

zT 

', 

b* 

DC 

S    9° 

/ 

180 

/ 

t- 

60 

50 

40 

30 

10 

/\    BUSY  HOUR  C 

Al 

LS  CARRIED  PEF 

TRUNK 

10      20      30  | 

ll      1      1      1      1      1 

0      200  400    600   800  1000 

BUSY  HOUR  CALLS  CARRIED  PER  GROUP  OF  TRUNKS 


Fig.  304. — Curve  A  shows  call-carrying  capacities  of  trunk  groups  of  various  sizes, 
manual  central  offices;  average  trunk  holding  time  of  120  seconds.  Curve  B  shows  call- 
carrying  capacity  of  each  trunk  in  groups  of  various  sizes. 


Trunking  Formulae. — Curve  A  Fig.  304  shows  the  call-carrying  capac- 
ities of  the  trunks  in  the  average  modern  manual  telephone  system. 
This  curve  follows  the  formula: 


Trunks  =  TC  +  4.2  a/(1  -  T)CT, 

in  which  C  is  the  number  of  calls  and  T  is  the  average  holding  time  in 
hours.  As  a  rule  a  trunk  should  not  be  expected  to  carry  over  15  or  18 
calls  during  the  busy  hour  even  between  large,  well-managed  manual 


TRAFFIC 


387 


offices,  while  between  smaller  offices  from  10  to  12  is  all  that  can  be 
expected. 

Reference  to  Fig.  305  shows  that  with  automatic  trunk  groups  a  much 
higher  carrying  capacity  is  experienced.  This  curve,  which  is  the  result 
of  thousands  of  observations  made  in  automatic  offices,  follows  the  empi- 
rical formula : 

Trunks  =  TC  +  3.785  (1  -  T)  \/cf. 

For  call  lengths  of  less  than  130  seconds  this  formula  may  be  written 
in  the  simplified  form 

Trunks  =  CT  +  3.7  \/TC. 

A  review  of  various  articles  published  in  recent  years  brings  out  the 
following  facts: 


feO 

55 
50 
45 
40 

k 

r 

0. 

§35 
o 

, 

A 

fr 

S  30 

3£ 

§25 
a 

t- 

20 

15 

10 

b 

BUSY 

HOUR 

CALLS 

CARRIED  PER  TRUNK 

r     l 

0      2 

0        3 

3       40      5|o 

0   100  200  300  400  500  600  700  800  900  1000 11001200  13001400  150016001700 1800  1900200C 

BUSY  HOUR  CALLS  CARRIED  PER  GROUP  OF  TRUNKS 

Fig.  305. — Curve  A  shows  call-carrying  capacities  of  trunk  groups  of  various  sizes 
between  automatic  central  offices;  average  trunk  holding  time  of  83  seconds.  Curve  B 
shows  call-carrying  capacity  of  each  trunk  in  groups  of  various  sizes. 


First :  That  a  fairly  fixed  relation  exists  between  the  number  of  busy- 
hour  calls  of  a  given  duration,  and  the  maximum  number  of  simultaneous 
calls  during  the  busy  hour. 

Second:  That  within  the  limits  of  holding  times  as  ordinarily  en- 
countered in  telephone  practice,  the  preceding  relation  holds  true  regard- 
less of  the  number  of  busy-hour  calls  and  the  average  holding  time,  so 
long  as  the  product  of  these  two  factors  remains  constant. 

Third:  That  the  average  calling  rate  of  a  large  group  of  subscribers 
usually  differs  somewhat  from  the  average  calling  rate  of  a  small  part  of 
that  group. 


388 


A  U  TO  MA  TIC  TELEPHON  Y 


While  all  of  the  methods  described  in  the  various  articles  are  very 
interesting,  since  the  ultimate  results  as  shown  in  the  chart  (Fig.  306)  do 
not  differ  materially,  it  will  not  be  necessary  here  to  go  into  details  regard- 
ing the  derivation  of  the  curves,  but  use  will  be  made  of  the  results  only. 


100 

95 
90 
85 

80 

7F 
70 
65 
60 
55 
50 
45 
40 
35 
30 
25 

zo 

15 


"0     5     10    15    20    25   30    35    40   45  50    55    60  "65  70   75    80   65  90   95  100 
T.C.(CoH  Hours) 

Fig.  306. — Busy  hour  average  calls  and  trunks. 

Let  a  equal  average  number  of  calls  during  period  (call  hours  TC). 

C  equal  average  calls  during  the  busy  hours. 

T  equal  average  holding  time  in  hour. 

X  equal  number  of  trunks. 

P  equal  probability. 
C  =  2.718  =  base  of  Naperian  logarithms. 

Campbell's  empirical  formula  X  =  TC  +  2.8  v'TC,  later  changed  to 
X  =  TC  +  3.7  v'CTC),  was  derived  from  observations  of  calls  handled 
by  groups  of  40  or  less  trunks.  If  X  =  10,  the  traffic  indicated  by  this 
formula  would  be  carried  with  a  probability  of  loss  of  1  call  in  100.  In 
the  practical  application  of  this  formula  to  larger  groups,  certain  per- 
centages have  been  added  that  have  not  been  taken  into  account  by  the 
various  commentators.  In  curve  E  this  formula  is  plotted  without  the 
percentage  factor. 


/ 

// 

W 

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i 

A 

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y/- 

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VR 

n  II 

a  a 

L 

'ES  AS: 

8~ Hiorred  trom  r - .  vui ana  r=.vi 
Tables  Prepared  be/  E.  C.  MOL  INA_ 

a 

4 

nd  Coinciding 
)pproximoition 

lotted  from  A 

uithA.K.ERLAHGS 
*     1       1       1       1 

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CURVEsl 

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rX.ERLANGS  Loss 

1 

Formula and  Practically  \        \ 
Co'inciolingwifhW.H.GRlNSTED'St-. 

Approximations    \ 

CURVE  "£"' 

-  Plotted  from  W.  LEE  CA 
Fmniric.nl  Formula 

MPBELLS 

TRAFFIC  389 


The  probability  that  in  a  certain  interval  there  will  be  various  numbers 
of  simultaneous  calls  may  be  represented  as  follows: 


0  calls  =  P„  =  -  — J-  =  e-a 

1  call  =  Px  =  -  — -2 -£-  -  =  ae-a 

a  +  a  +  5i  +  Si+ -) 

a2 

2'  i2c-a 

2  calls  =  P2  = g 3 =  '  oi 

0  +  "  +  il+|i+ °» 

X !  axe-a 

X  calls  =  Px  = 2 ir~  =    vt- 


X  or  more  calls  =  Px+  =  e~a  {  ^i  +  7v~~,~m  ^ 


\X!+  (X+  1)!  +  (X  +  2)! 

x+l  qX+2 


f     aA~ri 
More  than  X  calls  =  Pu+i)+  =  e~a  j  ,      1);  +  /X   ,   2) ; 

The  probability  that  in  certain  intervals  there  will  be  X  calls  in 
progress  in  a  group  limited  to  X  outlets,  is  expressed  by 

ax 
PLX  =  f]-  __ 

{l+a  +  ^  +  |-j+ fj 

This  latter  expression  more  nearly  represents  a  theoretical  measure 
of  the  lost  calls,  while  the  expression  for  Px  above  is  a  very  accurate 
approximation  and  lends  itself  to  ready  calculation.  This  is  shown  as 
curves  C  and  D  in  Fig.  306. 

The  results  for  the  expression  Px  +  are  in  more  general  use,  and  while 
perhaps  not  theoretically  representing  a  measure  of  the  lost  calls,  in 
providing  slightly  more  equipment  for  a  given  amount  of  traffic,  the 
error  is  on  the  safe  side.  This  is  shown  in  curves  A  and  B  (Fig.  306),  and 
tabulated  in  Table  A. 

For  our  purpose  the  values  of  curves  A  and  B  shown  on  the  chart  and 
in  Table  A  will  be  used.  These  values  have  been  taken  from  the  table 
appearing  in  an  article  by  Mr.  Molina,  of  the  American  Telephone  and 
Telegraph  Company,  published  in  the  American  Mathematical  Monthly 
of  June  1913. 


390  AUTOMATIC  TELEPHONY 


Trunks 


Table  A 
P  =  .001  P  =  .01 


TC  TC 

1  0.0010  0.010 

2  0.0454  0.149 

3  0.191  0.436 

4  0.429  0.823 

5  0.739  1.28 

6  1.11  1.79 

7  1.52  2.33 

8  197  2.91 

9  2.45  3.51 
10                                                 2.96  4.13 
12                                                 4.04  5.43 
14                                                 5.20  6.78 
16                                                 6.41  8.18 
18                                                 7.66  9.62 
20                                                 8.96  11.1 
30                                               15.9  18.7 
40                                               23.3  26.8 
60                                                 38.9  43.5 
80                                               55.2  60.7 

100                                                 71.9  78.2 

140  106.3  114.0 

180  141.4  150.3 

200  159.0  169.0 

300  249.0  261.0 

400  341.0  355.0 

500  434.0  450.0 

600  527.0  545.0 

700  621.0  640.0 

800  715.0  736.0 

900  810.0  832.0 

1000  905.0  '.)2S.O 

Arrangement  of  Multiple. — If  a  trunk  searching  element  is  limited  to 
10  outlets,  the  amount  of  traffic  handled  is  independent  of  the  order  of 
selection,  assuming  the  selection  to  be  instantaneous,  as  is  practically 
the  case.  That  is,  if  10  outlets  are  available  say,  to  100  selectors,  it  is 
immaterial  whether  the  outlets  are  tested  in  regular  rotation  begin- 
ning with  the  first  one  each  time,  or  whether  they  are  arranged  so  that  the 
first  10  selectors  will  begin  by  testing  the  first  outlet,  the  second  10  by  first 
testing  the  second  outlet  and  finishing  with  the  first,  the  third  by 
first  testing  the  third  outlet  and  finishing  with  the  second,  etc.  The  first 
arrangement,  however,  would  not  be  used  because  of  the  excessive  wear 
on  the  switch  connected  to  the  first  trunk  and  the  excessive  amount  of 
rotation  on  each  of  the  hundred  first  selectors,  since  if  the  traffic  indicated 
an  average  of  4.13  simultaneous  calls,  it  would  mean  that  half  the  calls 
would  be  extended  to  the  first  four  trunks,  and  for  the  remaining  half  the 
switches  would  rotate  over  at  least  four  contacts,  to  find  one  of  the  six 
remaining  trunks. 


Table   B 

Traffic  per 
Trunk 

Summation 
of  Traffic 

0.62 

0.62 

0.60 

1  22 

0.58 

1.80 

0.54 

2  34 

0.51 

2.85 

0.457 

3  307 

0.387 

3  694 

0.287 

3.981 

0.139 

4.120 

0.01 

4    13 

TRAFFIC  391 

Table  B  shows  the  amount  of  traffic  handled  per  trunk  in  a  group  of 
ten  when  the  trunks  are  selected  in  regular  rotation  beginning  from  the 
first.  These  figures  were  derived  from  the  1  in  100  column  of  Table  A  by- 
subtracting  adjacent  values. 

Order    of 

Selection 

1 
2 
3 

4 
5 
6 

7 
8 
9 
10 

From  the  foregoing  table  we  can  determine  the  probable  traffic- 
carrying  capacity  of  a  group  of  trunks  with  various  methods  of  bank 
multiplying.  Consider  case  1,  Fig.  307,  where  the  first  five  trunks  in 
each  group  would  handle  2.85  TC  and  the  common  group  would  handle 
1.28  TC  or  a  total  of  6.98  TC.  This  is  capacity  greater  than  that  of 
either  a  group  of  10  and  a  group  of  5,  or  1.5  groups  of  10,  but  not  as 
great  as  that  of  a  group  of  15.  Advantage  is  not  taken  of  this  increased 
capacity  in  the  direct  computation,  which  in  this  case  would  be  1.5 
groups  of  10,  but  the  arrangement  is  used  to  provide  an  additional  margin 
of  about  12^2  Per  cent  to  care  for  unforeseen  variations. 

In  case  2,  having  three  individual  groups  of  5,  and  a  common  group 
of  5,  the  20  trunks  would  carry  9.83  TC  or  about  20  per  cent  more  than 
two  groups  of  ten.  In  case  3,  the  30  trunks  would  handle  14.725  TC 
or  about  19  per  cent  more  than  three  groups  of  10.  In  case  4,  in  a  similar 
manner  it  may  be  computed  that  the  20  trunks  would  handle  9.83  TC 
or  about  20  per  cent  more  traffic  than  two  groups  of  10. 

CALCULATION  OF  AUTOMATIC  EQUIPMENT 

The  following  description  shows  how  the  traffic  data  are  used  to 
compute  the  quantities  and  distribution  of  the  various  elements  of 
switching  equipment  in  a  Strowger  automatic  exchange. 

Primary  Lines  witches. — Since  there  is  a  lines  witch  associated  with 
each  subscriber's  line,  the  number  of  working  primary  lineswitches  is 
determined  from  the  number  of  working  lines. 

When  a  subscriber's  intermediate  distributing  frame  is  interposed 
between  the  lineswitches  and  the  connector  banks,  so  that  any  lineswitch 
can  be  cross  connected  to  any  terminal  number,  it  is  necessary,  due  to 
circuit  requirements,  to  group  the  primary  lineswitches  in  classes  for  the 


392 


A  UTOMA  TIC  TELEPHON  Y 


following  services:  Flat  Rate,  Message  Rate,  and  Pay  Station  Rate. 
On  the  other  hand,  if  an  intermediate  frame  is  not  used  and  the  line- 
switches  are  permanently  connected  to  the  terminal  numbers,  then  the 
flat  rate  group  must  be  further  subdivided  into  individual  lines,  party 
line  and  P.B.X.  groups,  and  the  message  rate  group  must  also  be  sub- 
divided into  individual  line  and  P.B.X.  groups. 

GROUPING    OF     SELECTORS. 


1 


2       3      4      S 
I       I       I       1 


T  T  T  T  T 

G  7  8  &  IO 

I  2  A  A  5 

I  I  i  i  1 
6  7  6  3  IO 

i  i  i  i  1 

II  12  13  14  15 

i  I  i  i  1 


20  FIESTSEL. 
TO  B  TBUKJKS. 


KS      17      18      19      20 


30RBST.SQ-. 

TO 
20  TBUNKS. 


I  2  3  A  5  21      25    27    29    -3d 

i  i  i  i  1  i 

6  7  8  9  10  22 

i  I  1  1  i  i 

II  12  13  14  15  23    2£     28 

111111 

16  17  18  19  20  24 

i  i  T  i  i-  i 


40  REST  SO. 

TO 
30  TBUNKS. 


I        2       3      4       5       IS      17      18      19     20 


6       7      8       9       O 


^ 


II       12      13       14       15 


Fig.  307. 


@ 


GO  SELECTOES 

TO 
20  TEUKJKS. 


In  either  case  in  grouping  the  lineswitches  for  the  various  classes  of 
service,  precautions  must  be  taken  so  that  the  pay  station  lines  and 
message  rate  lines  do  not  have  access  to  the  same  first  selector  switches; 
otherwise,  under  certain  conditions,  there  would  be  premature  registra- 
tion of  meters  on  message  rate  lines,  due  to  the  circuit  requirements  of 
pre-payment  pay  station  service. 

The  number  of  primary  lineswitches  provided  for  the  initial  period 
for  each  class  of  service  is  usually  5  per  cent  greater  than  the  number  of 


TRAFFIC 


393 


working  lines  and  the   number  of   terminals   (connector  numbers)   is 
usually  10  per  cent  greater  than  the  number  of  working  lines. 

Grouping  of  Primary  Lineswitches. — It  is  recognized  that  the  average 
calling  rate  per  line  for  a  certain  class  of  service  is  based  on  the  originating 
traffic  on  a  thousand  or  more  lines,  the  calling  rate  for  any  small  groups, 
such  as  25,  50,  75  or  100  lines,  will  vary  from  this  average  rate.  However, 
if  the  average  calling  rate  is  based  on  a  smaller  number  of  lines,  there  will 
be  little  or  no  variation  when  considering  small  groups.  Table  C  shows 
the  percentage  variation  that  can  be  allowed  for  the  various  small  groups 
at  different  calling  rates: 

Table  C. — Average  Calling  Rate  per  Line 


Size  Group 

Less  than 

0.8 

0.8  to 

1.3 

1.3  to  2. 

M 

ORE  THAN  2 

25 

70 

60 

50 

40 

50 

50 

44 

37 

30 

75 

40 

35 

30 

25 

100 

35 

30 

25 

20 

Assuming  that  the  number  of  lineswitches  is  5  per  cent  greater  than 
the  number  of  working  lines,  Table  D  indicates  the  proper  grouping  of 
plunger  type  lineswitches  per  master  switch,  for  various  calling  rates  and 
with  a  holding  time  of  120  seconds. 


Table  D 


Primary  lineswitches  per  group 


Calling  rate  per  working  line 
Calling  rate  per  equipped  line 
Trunks  per  group 


0.85 
0.81 
0.9 


1.0 
0.957 
10.0 


1.1 

1.04 

9.0 


1.281.61 
1.22  1.53 

10.0    9.0 


1.89  2.1 

1.8    2.0 

10.0    7.0 


2.63  3.16 
2.5    3.0 
8.0    9.0 


3.72 
3.54 
10.0 


Ordinarily  the  average  traffic  would  not  be  so  great  as  to  necessitate 
groups  of  less  than  fifty  lines  per  master  switch,  except  in  unusual  cases, 
such  as  where  P.B.X.  trunks  or  other  flat  rate  lines  are  grouped  together. 
In  this  case  the  lines  of  heavy  traffic  can  be  isolated  and  grouped  25  per 
master  switch,  thus  reducing  the  average  calling  rate  of  the  remaining 
lines. 

If  local  secondary  lineswitches  are  not  used,  the  number  of  first 
selectors  would  correspond  to  the  number  of  equipped  outlets  from  the 
primary  lineswitches  as  explained  in  the  previous  paragraphs. 

First  Selectors  (with  Secondary  Lineswitches). — -When  each  of  the 
10  trunks  from  each  primary  sub-group  loads  to  a  secondary  lineswitch, 
which  in  turn  has  10  outlets  of  first  selectors,  then  each  primary  line- 
switch  will  have  access  to  100  first  selectors,  and  these  100  first  selectors 
function  as  one  group.  As  explained  in  the  following  section,  a  group  of 
100  outlets  reached  through  secondaries  is  not  fully  as  efficient  as  if 
reached  direct.     The  100  first  selectors  may,  however,  be  considered  as 


394  AUTOMATIC  TELEPHONY 

having  the  effectiveness  of  a  group  of  85.  If  we  consider  that  first 
selectors  are  sometimes  held  busy  by  line  "shorts"  or  "grounds"  or  by 
subscribers  unwittingly  leaving  their  receivers  down,  it  becomes  necessary 
to  allow  for  some  margin,  and  this  margin  is  obtained  by  using  the  1  in 
1000  probability.- 

From  Table  A  it  can  be  determined  that  a  group  of  85  trunks  will 
handle  59.4  TC  (call  hours)  with  a  probability  of  loss  of  1  in  1000. 
Therefore,  100  first  selectors  reached  through  secondaries  can  be  con- 
sidered to  carry  safely  59.4  TC  and  under  certain  conditions  it  has  been 
considered  that  satisfactory  service  could  be  given  on  the  basis  of  100 
first  selectors  carrying  75  TC.  On  the  basis  of  100  first  selectors  carrying 
59.4  TC  the  efficiency  of  the  switches  would  be  59.4  per  cent  and  the 
average  occupancy  per  switch  would  be  0.594. 

Local  Secondary  Lineswitches. — Experience  has  shown  that  good 
service  can  be  given  on  the  assumption  that  not  more  than  85  switches 
are  ever  in  use  simultaneously. 

In  order  to  minimize  the  difficulty  of  unavailable  trunks  from  primary 
switches  it  is  necessary  to  distribute  the  traffic  evenly  among  the  second- 
ary sub-groups,  and  the  following  three  methods  are  practiced  to 
contribute  toward  this  end: 

First:  A  special  scheme  of  cross  connection  between  primaries  and 
secondaries  so  that  whenever  there  is  a  tendency  for  the  primaries  to 
build  up  before  a  certain  sub-group,  the  next  call  attempted  will  break 
up  the  combination. 

Second:  The  "step-off"  relay  on  each  secondary  sub-group  is  so 
arranged  that  when  this  secondary  sub-group  has  a  predetermined 
number  of  calls  in  progress  on  successive  trunks,  all  of  the  primary 
switches  which  had  pre-selected  trunks  to  this  secondary  section  are 
stepped  off  to  another  sub-group. 

Third:  When  a  secondary  sub-group  has  all  trunks  busy,  all  of  the 
primary  master  switches  associated  with  that  entire  secondary  group, 
are  caused  to  operate  momentarily.  This  aligns  all  of  the  disengaged 
primary  lineswitches  on  the  shaft,  and  re-groups  the  primary  pre- 
selections. 

If,  from  each  primary  sub-group,  only  8  outlets  are  equipped,  even 
though  10  secondary  sub-groups  are  used  to  carry  the  traffic  to  100  first 
selectors,  there  would  not  be  a  theoretic  group  of  100  but  rather  one  and 
one-fourth  times  a  theoretic  group  of  80.  However,  there  is  so  little 
difference  between  the  two  that  a  theoretic  group  of  100  can  be  considered 
as  correct. 

To  compute  the  number  of  secondary  lineswitches  to  a  group  of  10 
sub-groups,  we  must  first  determine  the  number  of  primary  sub-groups 
required  to  load  the  group.  Consider  that  100  first  selectors  reached 
through   secondaries   will   handle   59.4    TC.     The    number   of  primary 


TRAFFIC  395 

sub-groups  to  bo  placed  in  one  group  may  be  computed  by  dividing 
this  by  the  average  traffic  offered  per  primary  sub-group  represented 
by  nTC  (n  being  the  number  of  working  lines  in  the  primary  sub-group, 
C  being  the  number  of  calls  per  line,  and  T  being  the  holding  time  in 
hours).  If  the  result  is  a  mixed  number,  then  only  as  many  primary 
sub-groups  should  be  grouped  together  as  are  represented  by  the  whole 
number,  unless  it  is  known  that  there  are  margins  in  the  original  data, 
which  would  permit  using  the  next  larger  whole  number.  After  this  has 
been  determined  the  average  occupancy  of  the  first  selectors  should  be 
determined  on  the  basis  of  actual  traffic  handled,  which  will  usually  be 
somewhere  near  59.4  TC. 

Rotary  Secondary  Lineswitches. — If  rotary  type  secondaries  are 
used,  each  group  is  made  up  of  10  sub-groups  having  25  outlets,  providing 
a  theoretic  group  of  250  outlets  or  an  effective  group  of  0.85  X  250  or 
212.5.  If  a  theoretic  group  of  250  is  too  large,  by  using  less  than  25 
outlets,  various  smaller  theoretic  groups,  such  as  240,  220,  or  200  may  be 
obtained. 

The  number  of  primary  sub-groups  required  to  furnish  traffic  for  a 
secondary  group  is  determined  much  as  in  the  case  of  plunger  type 
secondaries.  A  theoretic  group  of  250  trunks  or  an  effective  group 
of  212.5  will  handle  174  TC  (call  hours),  hence  from  50  to  60  primary 
sub-groups  will  be  required  to  keep  a  group  of  rotary  secondaries  busy. 
Present  practice  in  mounting  the  secondary  lineswitches  prohibits  the 
placing  of  more  than  50  primary  sub-groups  to  a  group. 

Second  Selectors. — From  the  traffic  data  we  can  obtain  the  per- 
centage distribution  of  the  originating  calls,  and  thus  determine  the  calls 
that  are  to  be  handled  by  second  selectors. 

A  group  of  10  trunks  will  handle  4.13  TC  with  a  probability  of  loss 
of  1  in  100.  Dividing  the  total  number  of  call  hours  to  be  handled  by 
4.13  will  give  the  number  of  groups  of  10  of  second  selectors. 

When  the  first  selectors  are  reached  through  secondary  lineswitches 
the  resulting  increase  in  efficiency  is  felt  beyond  the  first  selectors.  In 
other  words,  10  second  selectors  should  carry  more  traffic  than  4.13  TC. 

Suppose  in  an  office  that  had  just  sufficient  traffic  for  200  first  selectors 
(TC  =  2  X  59.4),  that  70  per  cent  of  the  total  traffic  (83.16  TC)  was 
local.  Dividing  this  by  4.13  would  indicate  a  need  for  slightly  more  than 
200  second  selectors.  Obviously  such  a  number  of  selectors  is  too  high 
since  it  would  be  more  than  the  number  of  first  selectors.  Accordingly 
some  allowance  can  be  made  for  the  greater  efficiency  of  second  selectors 
when  a  relatively  large  percentage  of  the  traffic  is  local. 

It  is  also  necessary  to  determine  the  method  of  multiplying  the  first 
selector  banks,  because  sometimes  the  number  of  second  selectors  must  be 
adapted  to  the  method  of  multiplying. 

First  Method.— The  preferred  method  is  to  determine  the  number  of 


396  AUTOMATIC  TELEPHONY 

second  selectors  working  in  groups  of  ten,  and  then  arrange  the  first 
selectors  so  that  each  group  will  have  a  number  of  second  selector  trunks 
individual,  and  the  remainder  common  to  two  or  more  groups.  For 
instance,  1 5  trunks  from  20  first  selectors  may  be  so  arranged  that  there 
will  be  five  trunks  individual  to  each  group  of  10  selectors  and  5  trunks 
common  to  the  entire  20.  If  the  arrangement  is  such  that  the  individual 
trunks  are  tested  first,  the  entire  group  will  handle  6.98  TC  with  a  prob- 
ability of  loss  of  1  in  100. 

Second  Method. — The  other  method  is  to  determine  the  occupancy 
of  the  first  selectors  for  the  particular  level  under  consideration,  and 
group  together  a  sufficient  number  of  first  selectors  on  the  basis  of  this 
occupancy,  to  offer  4.13  TC  to  the  second  selectors.  If  the  resultant 
number  of  first  selectors  is  not  a  multiple  of  five,  the  number  of  first 
selectors  is  cut  down  and  sufficient  trunks  are  equipped  to  handle  the 
traffic  from  such  reduced  group  of  first  selectors. 

Trunks  to  Other  Offices. — If  the  trunks  are  taken  directly  from  selec- 
tor levels,  the  number  of  trunks  is  determined  in  practically  the  same 
manner  as  the  number  of  second  selectors  just  described.  Since  the 
trunks  operate  through  cable  pairs  to  other  offices,  it  is  advisable  to 
provide  some  margin  to  allow  for  trunks  that  may  be  unavailable  because 
of  cable  conductors  being  in  trouble.  This  margin  is  usually  provided 
by  using  1  in  1000  probability  instead  of  1  in  100,  or  in  other  cases,  by 
using  the  1  in  100  table  and  adding  a  certain  percentage  to  the  resultant 
number  of  trunks. 

If  outgoing  secondary  lineswitches  are  used,  the  number  of  trunks  in 
the  group  is  determined  from  the  1  in  1000  probability  table,  on  the  basis 
of  the  theoretic  group  being  only  85  per  cent  available  as  explained 
for  first  selectors.  Either  plunger  type  or  rotary  type  lineswitches 
may  be  used  for  outgoing  secondaries. 

The  number  of  trunks  thus  determined  gives  also  the  number  of 
impulse  repeaters  required  at  the  originating  office,  and  the  number  of 
incoming  second  selectors  in  the  distant  office. 

Outgoing  Secondary  Lineswitches. — After  the  number  of  outgoing- 
trunks  has  been  determined,  as  just  outlined,  then  the  number  of  second- 
ary sub-groups  can  be  determined  on  the  basis  of  a  certain  number  of 
trunks  per  sub-group.  If  the  number  of  sub-groups  is  less  than  10,  each 
ten  trunk  group  must  be  divided  as  evenly  as  possible  among  all  secondary 
sub-groups  so  that  there  will  be  more  than  one  outlet  for  each  trunk 
group,  leading  to  each  secondary  sub-group.  This  results  in  somewhat 
lowered  efficiency,  since  if  a  certain  secondary  sub-group  is  momentarily 
or  permanently  busy,  two  or  more  outlets  would  be  busy  to  the  first 
selectors.  In  such  cases  the  10  trunks  are  considered  at  only  85  per  cent 
of  their  theoretic  efficiency,  and  will  handle  0.85  X  4.13  or  3.51  TC. 

The  number  of  outgoing  secondary  lineswitches  can  be  determined  on 


TRAFFIC  397 

the  basis  of  10  outlets  handling  4.13  TC  or  3.51  TC  as  the  case  may  be, 
and  then  grouping  the  first  selectors  so  that  each  group  has  a  certain 
number  of  trunks  individual  to  the  group,  and  the  remaining  trunks 
common  to  two  or  more  groups;  or,  as  in  the  case  of  determining  the 
number  of  second  selectors,  the  occupancy  of  the  first  selectors,  for  the 
particular  level  under  consideration  is  determined  and  the  first  selectors 
arranged  in  groups  so  as  to  offer  4.13  TC  or  3.51  TC  per  group.  If  the 
resultant  number  of  first  selectors  is  not  a  multiple  of  five,  the  number  of 
first  selectors  is  cut  down  and  a  sufficient  number  of  outlets  are  equipped 
to  care  for  the  traffic  offered  by  the  reduced  number  of  first  selectors. 

Third  Selectors. — The  third  selectors  in  a  five-digit  system  are  used 
for  distributing  the  calls  to  the  various  hundred  groups  in  each  thousand. 
Third  selectors  working  in  groups  of  ten  can  handle  4.13  TC.  Accord- 
ingly, the  number  of  groups  of  ten  required  for  each  thousand  may  be 
computed  by  dividing  the  total  amount  of  traffic  to  be  handled  by  each 
level,  by  4.13  TC.  The  second-selector  banks  should  then  be  so  multi- 
pled  as  to  keep  these  groups  of  ten  uniformly  busy. 

In  case  the  scheme  of  multiplying  which  is  used,  is  such  that  the 
local  traffic  is  kept  separate  from  the  incoming  traffic,  because  of  the 
fact  that  the  incoming  switches  may  be  reached  through  outgoing  second- 
aries at  the  originating  office,  they  will  handle  traffic  at  a  different  rate. 
Therefore,  the  occupancy  per  second  selector  for  each  level  will  have 
to  be  determined  for  both  incoming  and  local  switches,  and  the  proper 
number  of  switches  combined  to  supply  ten  trunks.  The  resultant 
grouping  of  second  selectors  should  be  in  multiples  of  ten. 

Connectors. — The  number  of  connectors  is  computed  by  first  de- 
termining the  average  calls  terminating  in  each  hundred  and  applying  a 
percentage  for  variation  as  shown  in  Table  C.  Then  if  the  traffic  is  less 
than  10  trunks  will  carry  we  determine  from  the  equivalent  TC  value  the 
number  of  connectors  from  the  second  column  of  Table  A.  If  the  number 
of  calls  is  greater  than  10  trunks  will  carry,  then  by  dividing  the  number 
of  calls  by  the  number  that  10  trunks  will  carry,  the  necessary  number  of 
groups  of  10  may  be  found.  The  third  selector  multiple  may  then  be 
arranged  so  there  will  be  individual  and  common  trunks.  If  this  is  not 
done,  the  number  of  connectors  must  be  determined  on  the  basis  of  full 
groups  of  10  and  a  remaining  partial  group. 

To  take  care  of  toll  calls  from  toll-selector  banks,  combination  toll 
and  regular  connectors  can  be  provided.  These  should  be  arranged  so 
that  they  are  the  last  choice  for  regular  calls,  so  as  to  secure  maximum 
availability  for  toll  calls. 

Rotary  Connectors. — The  method  of  determining  the  number  of  these 
connectors  is  the  same  as  for  regular  connectors.  Due  to  the  fact  that 
the  calling  rate  to  P.B.X.  rotary  groups  is  much  heavier  than  to  regular 
groups,  and  that  the  calling  rate  given  is  usually  the  result  of  observation 


398 


A  UTOMA  TIC!  TELEPHON  Y 


of  this  particular  class  of  calls,  it  is  not  necessary  to  add  much,  if  any, 
percentage  for  variation. 

Combination  rotary  connectors  are,  as  a  rule,  rather  complicated 
and  there  is  therefore,  little  economy  in  providing  them.  Accordingly, 
separate  toll  rotary  connectors  are  provided  and  their  number  is  com- 
puted in  a  similar  manner  as  for  local  connectors. 

Summary. — In  the  first  method  the  number  of  trunks  is  determined 
on  the  basis  of  independent  groups  of  ten,  but  the  resultant  number  of 
switches  is  so  arranged  by  means  of  overlapping  multiples  or  common 
groups  of  trunks  as  to  afford  team  work  between  groups.  This  extra- 
call  handling  capacity  is  used  as  a  margin  to  care  for  possible  peak  and 
emergency  traffic.  This  is  essential  if,  as  in  this  case,  the  busy  hour 
originating  traffic  is  the  basis  of  distribution,  since  the  peak  traffic  in  any 
given  direction  may  not  coincide  with  the  peak  for  the  originating  traffic. 

In  the  second  method,  each  group  of  trunks  is  independent  of  all 
other  groups  and  is  accessible  only  from  a  fixed  field.  There  is  no  team 
work,  and  each  group  must  be  arranged  to  take  care  of  its  own  peak. 

A  sample  calculation  was  worked  out  for  a  four-office  exchange  com- 
puting the  amount  of  equipment  for  one  of  the  offices.  Without  going 
into  details,  for  which  we  have  not  space,  the  equipment  required  is  shown 
in  the  following  summary,  which  compares  the  first  and  second  methods 
of  computing  the  number  of  selectors. 

Summary 


First  method 


Number 


Remarks 


Second  method 


Number 


Remarks 


Primary  lineswitches 

Secondary  lineswitches. .  . 

First  selectors 

Local  second  selectors 
Outgoing  sec.  L.  S.  to  B. 

Repeaters  to  B 

Outgoing  sec.  L.  S.  to  C. . 

Repeaters  to  C 

Repeaters  to  D 

Incoming  2d  sel.  B 

Incoming  2d  sel.  C 

Incoming  2d  sel.  D 

Third  selectors 

Non-rotary  connectors .  .  . 
Combinations  connectors 

Rotary  connectors 

Toll-rotary  connectors    .  . 


6300 
1260 
600 
360 
300 
130 
300 
200 

45 
145 
185 

50 
850 
640 
256 
180 

30 


30%  margin 
30%  margin 
10%  margin 
16%  margin 
10%  margin 
10%  margin 


15%  margin 

15%  margin 

9%  margin 


6300 
1260 
600 
360 
200 
130 
300 
200 

48 
145 
1S5 

50 
879 
640 
256 
222 

30 


10%  margin 
10%  margin 


8%  margin 
8%  margin 


Reasons  for  High  Efficiency  of  Automatic  Trunks. — One  reason  for 
the  increased  efficiency  of  automatic  trunks  is  found  in  the  shorter  time 


TRAFFIC 


399 


per  connection.  It  is  generally  considered  in  manual  practice  that  the 
average  length  of  connection  is  two  minutes  where  the  "B"  operator 
uses  key  ringing;  whereas  many  thousands  of  observations  made  in  auto- 
matic plants  show  an  average  length  of  connection  not  exceeding  83 
seconds.  It  is  difficult  to  account  for  all  of  the  difference  between  a 
holding  time  of  120  seconds  and  one  of  83  seconds. 

The  differences  in  the  times  required  for  setting  up  connections,  con- 
versation and  disconnecting  are  shown  graphically  in  Fig.  308.  It  will 
be  noted  that  the  average  subscriber  to  automatic  service  answers  his 
telephone  quicker  than  the  average  manual  subscriber.  This  is  especially 
noticeable  when  key  ringing  is  used  on  the  manual  switchboard.  It  is 
conceivable  that  the  average  length  of  conversation  in  automatic  plants 
is  somewhat  shorter,  because  of  the  comparative  ease  with  which  a  con- 
nection mav  be  reestablished. 


A  B 
*&    toiJi 


MANUAL  TRUNKS-KEY 


|A  B  C 


RINGING  BY  B  OPERATORS 
84  X     I 


MANUAL  TRUNKS  - 
I    84M 


AUTOMATIC  RINGING 


AUTOMATIC  TRUNKS 
86  M 


D  E 


3^  2% 


-J-+ 


Fig.  308. — Graphical  analysis  of  trunk  holding  times:  A,  is  instant  at  which  a  trunk  ia 
assigned  by  a  "B"  operator  or  taken  by  an  automatic  selector  switch;  B,  connection 
completed  and  ringing  started;  C,  subscriber  answers;  D,  Conversation  completed — dis- 
connect signals  given  to  "^4."  operator;  E,  "A"  operator  or  automatic  selector  disconnects 
from  the  trunk;  F,  "5"  operator  releases  the  trunk. 


Part  of  the  time  designated  as  the  conversation  period  on  manual 
trunks  elapses  after  conversation  has  really  ceased  and  before  the  dis- 
connect signals  of  both  subscribers  have  reached  the  "i"  operator.  An 
automatic  connection  releases  when  the  calling  party  or  operator  discon- 
nects. If  a  called  manual  subscriber  is  connected  to  a  private  branch  ex- 
change, which  is  frequently  the  case  in  the  business  districts  of  cities, 
there  is  always  more  or  less  loss  of  time  between  the  instant  at  which  the 
called  party  hangs  up  his  receiver  and  that  at  which  the-  P.B.X.  operator 
pulls  down  the  connection  and  thereby  gives  the  disconnect  signal  to  the 
"A  "  operator. 

Quite  a  large  percentage  of  calls  result  in  the  calling  party  securing  the 
busy  signal  or  no  response  from  the  called  party.  In  fact  about  12  per 
cent  of  the  former  and  16  per  cent  of  the  latter  are  included  in  the  general 
average.  Consequently  since  the  busy  signal  is  secured  more  promptly 
in  automatic  systems,  especially  in  multi-office  areas,  and  since  an  auto- 
matic subscriber  does  not  wait  on  a  "don't  answer"  call  until  an  operator 
tells  him  that  the  wanted  party  does  not  respond,  but  hangs  up  his  re- 


4()()  AUTOMATIC  TELEPHONY 

ceiver  of  his  own  initiative  both  of  these  types  of  ineffective  calls  help  to 
reduce  the  holding  time  of  the  average  call. 

Calls  to  "Long  Distance,"  "Information,"  "Complaint,"  etc.,  are  all 
shorter  in  automatic  systems  because  the  calling  party  secures  a  direct 
connection  (very  often  by  one  turn  of  the  dial)  to  the  desired  official, 
instead  of  waiting  for  an  "A  "  operator  to  give  him  the  connection. 

The  disconnection  is  accomplished  much  more  quickly  in  the  automa- 
tic system  and  this  feature  is  especially  helpful  during  the  busy  moments 
when  manual  operators  are  rushed  and  consequently  comparatively  slow 
in  pulling  down  connections.  The  interval  of  time  that  elapses  between 
release  of  a  trunk  by  one  automatic  selector  and  seizure  of  it  by  another 
need  be,  and  often  is,  but  a  fraction  of  a  second.  This  too  increases  the 
efficiency  of  the  trunks. 

Variations  in  Holding  Times. — The  average  holding  time  per  connec- 
tion may  be  quite  different  in  one  city  from  what  it  is  in  another  and  it  is 
generally  shorter  in  a  business  office  than  in  a  residence  office. 

In  comparison  with  the  average  call  length  of  83  seconds,  the  following 
results,  of  observations  made  on  the  private  automatic  exchange  of  a 
busy  factory,  are  interesting. 

Time  required  to  complete  connection 3.5  sees. 

Time  required  for  called  party  to  answer 8.1  sees. 

Time  spent  in  conversation 30 . 1  sees. 

Time  required  for  disconnection 1.0  sees. 

Total  average  time 42 . 7  sees. 

In  this  system  a  group  of  ten  trunks  handled  a  traffic  that  sometimes 
ran  up  to  300  busy-hour  calls,  without  being  overloaded,  while  Fig.  305 
shows  that  where  the  length  of  connection  is  83  seconds,  a  group  of  ten 
trunks  should  not  be  expected  to  carry  over  180  busy-hour  connections. 
This  variation  emphasizes  the  need  of  a  knowledge  of  the  average  length 
of  connections  in  determining  trunk  group  capacities. 

Two-way  Trunks. — Another  point  that  has  been  observed  in  traffic 
studies  and  which  should  be  considered  here,  is  that  the  peak  of  the  load 
on  the  trunks  outgoing  from  one  office  A  to  another  office  B  may  not 
occur  at  the  same  hour  of  the  day  as  the  peak  of  the  load  on  the  trunks 
incoming  from  B  to  A.  This  is  sure  to  be  true  if  A  is  a  business  office 
and  B  is  a  residence  office.  It  is  readily  understood,  therefore,  that  if 
the  same  trunks  carried  connections  in  either  direction,  as  required,  that 
greater  trunk  efficiency  would  be  secured.  Two-way  trunks  are  now 
used  to  a  very  limited  extent  in  automatic  practice  and  it  is  probable  that 
they  will  eventually  be  used  much  more,  although  they  are  not  considered 
practicable  in  common-battery  manual  systems  between  central  offices. 

Need  of  Traffic  Studies.— It  has  not  been  customary  in  automatic 
central  offices  to  keep  up  the  constant  detailed  study  of  traffic  that  is 


TRAFFIC  401 

made  in  well-regulated  manual  offices,  where  frequent  peg  counts  and 
plug  counts  are  taken  for  several  reasons: 

1.  To  secure  intimate  knowledge  of  traffic  not  only  during  the  busy 
hour  but  throughout  the  entire  day  from  5  a.m.  to  11  p.m.  so  that  at  all 
times  the  number  of  operators  may  be  kept  adjusted  as  closely  to  the 
load  curve  as  practicable.  Failure  to  do  this  would  usually  result  either 
in  slow  service  on  part  or  all  of  the  positions  at  times  or  in  a  decided  loss 
in  wages  paid  to  unnecessary  operators. 

2.  To  distribute  the  load  evenly  among  the  operators,  thus  requiring 
each  to  do  her  share  of  the  work,  and  ensuring  every  subscriber  at  tent  ion 
as  prompt  as  that  received  by  others. 

With  automatic  switchboard  equipment  the  conditions  are  different. 
The  first  reason  for  traffic  study  in  a  manual  exchange  does  not  apply  to 
an  automatic  at  all,  because,  since  the  connections  are  made  by  machines, 
it  is  impracticable  to  eliminate  idle  machines  during  the  less  busy  hours 
or  to  secure  any  economy  by  doing  so.  The  second  reason  docs  not  apply 
either.  All  calls  passing  over  a  group  of  trunks  are  distributed  among 
them  automatically.  It  is  impossible  for  any  machine  to  shirk  its  share 
of  the  load.  It,  therefore,  is  necessary  only  to  be  sure  that  the  proper 
number  of  machines  are  installed  in  each  section  of  the  switchboard  to 
carry  the  peak  load  safely.  This  can  be  most  readily  determined  without 
a  call  count  by  having  the  switchboard  attendants  watch  for  overloaded 
and  underloaded  sections. 

Distributing  the  Load. — A  lack  of  trunks  may  be  remedied  in  eit fun- 
one  of  two  ways. 

By  shifting  subscribers'  lines  from  an  overloaded  unit  to  an  under- 
loaded one. 

By  increasing  the  number  of  trunks  from  or  to  the  overloaded  unit. 

Formerly  it  was  the  practice  to  follow  unvaryingly  the  second  met  hod, 
because  it  was  known  that  most  operating  companies  at  that  time  did  not 
wish  to  incur  the  expense  of  line  intermediate  distributing  frames,  which 
the  first  plan  requires.  It  was  even  sometimes  the  practice  to  install  a 
special  unit  to  which  very  busy  lines  were  transferred  from  an  overloaded 
unit,  without  changing  the  subscriber's  number. 

Recent  years  have  seen  an  increasing  use  of  the  line  intermediate 
distributing  frame,  for  the  flexibility  which  it  affords  is  appreciated. 

Whether  the  trunks  outgoing  from  any  lineswitch  unit  lead  to  secon- 
dary lineswitches  or  to  first  selectors  it  is  good  practice  to  pass  them 
through  an  intermediate  distributing  frame  which  will  enable  their 
number  or  distribution  to  be  readily  changed.  It  is  not  a  very  difficult 
matter,  however,  to  increase  the  number  of  these  trunks  without  an 
intermediate  frame.  To  prepare  for  a  proper  redistribution  of  the  trunks 
busy-hour  observations  should  be  made  on  less  busy  sections  of  the  switch- 
board to  determine  where  the  number  of  trunks  may  be  reduced  safely. 


I  ( )2  A  U  TOM  A  TIC  TELEPHON  Y 

For  example,  if  there  are  ten  selector  switches  in  a  given  group  and  an 
attendant  who  has  made  frequent  busy-hour  observations  personally 
or  by  means  of  meters,  has  never  found  more  than  six  in  use  at  any  one 
time  it  may  be  assumed  that  no  more  thaoi  seven  or  eight  are  needed. 
Some  managers  advise  removing  the  extra  switches  from  service,  whether 
they  are  needed  elsewhere  or  not,  in  order  to  reduce  the  number  of 
machines  to  be  kept  in  good  working  order  as  much  as  possible.  They 
contend  that  while  the  switches  might  be  used  sometimes  when  there  is  a 
great  rush  of  calls  due  to  a  large  fire  or  other  events  of  great  public  inter- 
est, it  is  impossible  to  care  for  all  calls  in  such  an  emergency  anyway, 
whether  the  switchboard  is  manual  or  automatic.  Of  course  if  it  is 
impracticable  to  reduce  the  number  of  trunking  switches  in  any  section, 
the  overburdened  section  must  be  relieved  by  the  addition  of  new  trunk- 
ing switches.  Since  automatic  switchboards  are  built  in  small  sections 
it  is  generally  a  very  simple  matter  to  install  additional  facilities. 

Distributing  the  Load  at  Time  of  Installation. — An  uneven  distribu- 
tion of  the  load  among  the  primary  lineswitch  units  can  be  avoided  to  a 
large  extent  by  a  proper  system  of  distributing  the  lines  to  the  units  at 
the  time  the  switchboard  is  first  put  into  service,  and  as  additional  lines 
are  added  to  it.  The  subscribers  should  be  divided  first  into  two  classes, 
namely,  business  and  residence.  The  residence  lines  should  then  be 
distributed  equally  among  the  lineswitch  units.  The  business  lines 
should  be  further  subdivided  into  classes  indicated  by  the  kinds  of  business 
for  which  they  are  used.  Lines  to  grocers  and  markets  should  be  put 
into  one  class;  lines  to  stock  brokers,  banks,  etc.,  in  another;  lines  to 
wholesale  and  commission  merchants  in  another;  lines  to  railroad  offices 
in  another,  etc.  The  lines  in  these  sub-classes  should  then  be  distributed 
among  the  lineswitch  units.  Of  course  the  houses  which  have  more  than 
one  line  under  the  same  number  must  be  assigned  to  the  units  especially 
equipped  with  trunking  connector  switches  for  handling  multiple  lines 
and  private-branch-exchange  trunks. 

Each  class  of  lines  has  its  rush  hour.  For  example,  housekeepers  are 
calling  their  grocers  and  markets  in  the  early  forenoon  but  are  not  doing  so 
to  any  extent  in  the  middle  of  the  day,  consequently  the  distribution  of 
each  class  of  calls  among  the  lineswitch  units  is  an  excellent  preventative 
of  unevenly  loaded  units,  at  any  hour  of  the  day. 

Note. — The  reader  who  wishes  to  pursue  this  subject  further  is  referred  to  the 
following  bibliography: 
W.  Lee  Campbell:  "A  Study  of  Multi  Office  Automatic  Switchboard  Telephone 

Systems."     A.  I.  E.  E.  Proceedings,  1908. 
F.    Johannson:    "Telephone    Management   in   Large    Cities."     Published    in   Post 

Office  Electrical  Engineers'  Journal,  October,  1910. 
E.  C.  Molina:  "The  Probability  of  an  Event  Happening  at  Least  c  Times  in  n  Trials." 

American  Mathematical  Monthly,  June,  1913. 


TRAFFIC  403 

F.  Spieker:  "Die  Abhangigkeit  des  erfolgreichen  Fernsprechanrufes  von  der  Anzahl 
der  Verbindungsorgane."     Published  by  Julius  Springer,  Berlin,  1913. 

W.  Lee  Campbell:  "Traffic  Studies  in  Automatic  Switchboard  Telephone  Systems  " 
A.  I.  E.  E.  Proceedings,  1914. 

P.  V.  Christensen:  "The  Number  of  Selectors  in  Automatic  Telephone  Systems." 
Published  in  Post  Office  Electrical  Engineers'  Journal,  October,  1914. 

Fritz  Lubberger:  "Die  Anpassung  der  Fernsprechanlager  an  die  Verkehrschwank- 
ungen."     Published  by  Julius  Springer,  Berlin,  1914. 

W.  H.  Grinsted:  "A  Study  of  Telephone  Traffic  Problems."  Published  in  Post 
Office  Electrical  Engineers'  Journal,  April,  1915. 

M.  Milon:  "Determining  the  Number  of  Selectors  Needed  in  an  Automatic  Tele- 
phone System."     Translated  in  Telephone  Engineer,  May,  1917. 

F.  R.  McBerty:  "Machine  Switching  Telephone  Gear."     Abstracted  in  Telephone 

Engineer,  May,  1917. 

A.  K.  Erlang:  "Solution  of  Some  Problems  in  the  Theory  of  Probability  of  Sig- 
nificance in  Automative  Telephone  Exchanges."  Published  in  Post  Office 
Electrical  Engineers'  Journal,  January,  1918. 

W.  H.  Grinsted:  "Theory  of  Probability  in  Telephone  Problems."  Published  in 
Post  Office  Electrical  Engineers'  Journal,  October,  1918. 

G.  F.  O'Dell:  (British)  Post  Office  Electrical  Engineers'  Journal,  October,  1920. 
Fred  L.  Baer:  "Computation  of  Quantities  of  Telephone  Switching  Equipment." 

Presented  before  Western  Society  of  Engineers,  Nov.  4,  1920. 


CHAPTER  XVII 
DEVELOPMENT  STUDIES 

Definition  and  Object. — The  term  "Development  Study"  is  applied 
to  the  study  that  should  be  made  in  any  city  in  which  it  is  proposed 
to  establish  a  telephone  system,  or  in  any  city  where  extensive  additions 
or  betterments  to  the  existing  system  are  contemplated.  The  object  of 
the  study  is  to  determine  the  probable  requirements  for  telephone  ser- 
vice of  the  city,  and  the  approximate  location  of  such  requirements, 
for  both  the  present  and  the  future,  so  that  all  work  executed  for 
extensions  and  betterments  will  be  planned  to  provide  telephonic  facili- 
ties at  the  minimum  cost,  consistent  with  good  engineering,  and  all  mate- 
rial entering  into  the  providing  of  facilities  will  be  properly  located  and 
of  a  character  to  serve  during  its  normal  efficient  life. 

The  Fundamental  Plan. — The  result  of  such  a  study  is  a  comprehen- 
sive plan  for  all  initial  and  future  work.  The  result  is  customarily 
referred  to  as  the  "Fundamental  Plan"  of  the  city. 

In  order  to  provide  telephonic  facilities  to  a  city  line,  a  pair  of  wires  is 
required  from  the  subscriber's  station  to  the  central  office.  A  main-line 
subscriber  obviously  requires  one  pair  for  his  exclusive  use,  while  a  party- 
line  subscriber  shares  a  pair  of  wires  with  others,  the  number  of  subscribers 
being  determined  by  the  number  of  stations  that  class  of  service  will 
permit  to  be  connected  to  a  single  pair,  and  also  by  the  number  that  can 
be  attached  in  actual  practice.  Under  average  conditions  a  four-party 
line  will  serve  2.8  subscribers  and  a  two-party  line  1.3  subscribers-  It 
will  be  seen,  therefore,  that  in  making  a  development  study  the  number 
of  lines  is  the  important  factor,  and  the  number  of  telephones  merely  an 
aid  in  learning  the  number  and  approximate  location  of  the  required  lines. 

The  following  are,  therefore,  the  main  points  to  be  determined  by  a 
development  study. 

1.  The  present  and  ultimate  number  of  lines  necessary  to  take  care  of 
the  present  and  ultimate  possibilities. 

2.  The  most  economical  location  of  offices  for  the  present  and  ultimate 
requirements. 

3.  The  ultimate  underground  conduit  plan. 

Inasmuch  as  the1  average  life  of  material  entering  into  the  construction 
of  a  telephone  system  is  approximately  15  years,  the  ultimate  require- 
ments are  usually  referred  to  as  the  requirements  15  years  later  than 
the  date  of  the  development  study.     It  must  be  borne  in  mind  that  a  de- 

404 


DEVELOPMENT  STUDIES  405 

velopment  study  is,  in  reality,  a  study  for  a  given  number  of  lines  in  a 
designated  community  and  the  results  of  the  study,  or  the  fundamental 
plan,  will  hold  if  the  estimated  ultimate  is  reached  a  few  years  prior  or 
subsequent  to  the  15-year  period. 

The  practical  method  of  determining  the  present  and  ultimate  possi- 
bilities is  by  an  actual  count  of  the  present  possibilities  in  each  block  of  the 
city.  The  possibilities  should  be  classified  according  to  the  kinds  of 
service  furnished  and  translated  into  the  required  number  of  lines  or 
pairs. 

While  a  study  of  the  present  number  of  lines  in  use  and  their  location 
may  be  of  value  in  making  a  development  study  of  a  city  with  an  existing 
system,  it  should  not  be  taken  for  granted  that  this  satisfies  the  require- 
ments unless  there  is  at  least  one  telephone  for  every  eight  inhabitants 
in  an  average  American  city,  in  which  practically  everybody  is  white. 
Where  a  large  portion  of  the  population  belongs  to  the  negro  race,  or  a 
considerable  portion  of  the  white  population  is  made  up  of  very  poor 
workers  in  factories,  the  requirements  will  be  less.  In  some  cities  one 
telephone  to  15  inhabitants  is  all  that  can  be  expected. 

The  number  of  possibilities  in  each  city  block  is  usually  determined  by 
what  is  termed  a  "house  count"  and  is  taken  by  making  a  block-by-block 
study  of  the  city  and  determining  from  the  classes  of  the  buildings  and 
their  uses,  the  probable  telephone  requirements  of  the  occupants.  The 
detailed  methods  for  taking  such  a  count  for  either  an  automatic  or  a 
manual  system  have  been  repeatedly  discussed  in  various  publications 
and  will  not  be  repeated  here. 

After  these  data  have  been  gathered  they  should  be  transferred  to  a 
good-sized  map  of  the  city.  On  each  block  of  this  map  should  be  indicated 
the  number  of  lines  necessary  to  provide  service  for  prospective  patrons. 
When  the  map  showing  the  immediate  lines  is  completed,  then  a  study 
should  be  made  of  the  probable  growth  of  the  city  both  in  numbers  and 
in  area  during  the  next  15  years. 

If  there  is  a  well-defined  tendency  for  the  city  to  grow  in  some  particu- 
lar direction,  this  should  be  discovered  and  a  map  should  be  made  showing 
the  expected  number  and  locations  of  the  necessary  lines  at  the  end  of  the 
15-year  period. 

When  this  map  has  been  completed  and  all  data  on  the  existing  plant 
are  at  hand,  the  determination  of  the  central  office  location  or  locations 
may  be  undertaken. 

First,  the  study  of  the  best  central  office  locations  at  the  end  of  a 
15-year  period  should  be  made,  and  then  the  best  arrangement  for 
immediate  needs  should  be  compared  with  it  and  a  plan  thus  worked  out 
which  will  not  only  satisfy  present  needs,  but  which  may  be  extended  and 
enlarged  year  by  year  without  wasteful  rearrangement.  When  making 
this  plan,  the  equipment  already  installed,  if  any,  should  be  fully  consid- 


406  AUTOMATIC  TELEPHONY 

cred,  and  those  portions  of  it  which  are  worth  while  should  be  incorpo- 
rated in  the  new  plan. 

DEVELOPMENT  STUDIES  FOR  AUTOMATIC  TELEPHONE  SYSTEMS 

What  has  been  said  in  the  foregoing  portion  of  this  chapter  is  as  appli- 
cable to  a  manual  system  as  to  an  automatic,  and  it  therefore  will  not  be 
enlarged  upon,  but  we  have  now  come  to  the  parting  of  the  ways,  because 
automatic  equipment  practice  may  indicate  that  a  multi-office  system 
should  be  installed  where  manual  practice  would  justify  nothing  but  a 
single-office  system.  The  planning  of  a  multi-office  automatic  system 
not  only  differs  from  the  planning  of  a  multi-office  manual  system,  but 
the  task  should  be  approached  with  an  entirely  different  attitude  of 
mind. 

While  in  manual  practice,  systems  serving  large  cities  are  divided  up 
to  save  cable  and  conduit,  division  of  a  system  of  less  than  10,000  lines  is 
generally  regarded  as  undesirable  and  to  be  avoided,  if  it  is  practicable  to 
do  so.  It  is  therefore  the  general  practice  in  the  smaller  cities  to  carry 
all  or  the  bulk  of  the  traffic  on  one  large  switchboard,  branch  offices  being 
installed  under  sufferance  and  only  for  the  most  urgent  reasons.  An 
engineer  laying  out  an  automatic  plant  should  realize  that  while  this 
antipathy  toward  dividing  offices  of  10,000  lines  or  less  is  reasonable  in 
manual  practice,  it  is  not  reasonable  in  automatic  practice.1 

The  first  cost  of  any  telephone  plant  may  be  divided  readily  into  three 
principal  items: 

1.  Cost  of  the  apparatus  (both  central  office  and  subscriber's  station). 

2.  Cost  of  the  central  office  buildings  and  furnishings. 

3.  Cost  of  the  wire,  cable  and  conduit  plant. 

The  last  of  the  items  named  is  the  largest  of  the  three.  It  is  a  variable 
quantity  affected  by  the  number  of  lines  and  offices  in  the  system,  the 
kind  of  soil,  character  of  pavements,  the  density  of  population,  form  of  the 
city,  obstructions  such  as  lakes,  rivers,  etc.  Generally,  however,  it 
will  amount  to  more  than  the  other  two  items  combined  and  it  is  not  rare 
for  it  to  be  two-thirds  of  the  entire  first  cost. 

ECONOMIC  WASTE  IN  TELEPHONE  SYSTEMS  AND  ITS  REMEDIES 

Considering  this,  it  is  hard  to  realize  that  in  the  face  of  all  the  progress 
which  has  been  made  in  the  development  of  the  telephone  art  the 
efficiency  of  the  cable  and  conduit  is  still  so  small  that  in  most  systems  at 
least  nine-tenths  of  the  subscribers'  lines  are  idle  even  at  the  peak  of  the 
load.  In  the  chapter  on  Traffic  is  shown  a  list  of  results  of  observations 
which  indicate  that  in  automatic  offices  of  8000  to  10,000  lines,  handling 

1  See  paper  entitled  "A  Study  of  Multi-office  Automatic  Switchboard  Telephone 
Systems"  presented  by  Mr.  W.  Lee  Campbell  before  the  American  Institute  of 
Electrical  Engineers  at  the  Annual  Convention,  June  29— July  2,  1908. 


DEVELOPMENT  STUDIES  407 

a  heavy  load,  the  maximum  number  of  conversations  taking  place  at  the 
busiest  moment  does  not  equal  4  per  cent  of  the  number  of  lines  in  service. 
Since  each  conversation  requires  two  lines  these  figures  show  less  than  8 
per  cent  of  the  lines  in  actual  use  for  conversation,  operating  or  signalling 
at  the  peak  of  the  load. 

To  reduce  the  great  economic  waste  represented  by  the  90  per  cent  of 
the  costly  cable  and  conduit  equipment  which  is  idle  even  at  the  busiest 
moment,  two  remedies  have  been  applied  to  some  extent,  viz.,  party 
lines  and  multiplication  of  offices. 

The  first  is  but  a  poor  and  inadequate  remedy  at  best  and  not  appli- 
cable to  the  lines  of  busy  subscribers. 

The  second  remedy  offers  more  hope  and  is  capable  of  much  develop- 
ment by  judicious  engineering.  The  unattended  sub-office  has  a  large 
field  of  usefulness  in  the  growing  part  of  a  city,  where  it  is  not  certain 
just  how  thickly  settled  it  will  untimately  become.  If  the  region  fails 
to  develop  beyond  a  certain  point,  then  let  the  sub-office  remain.  If 
the  real  estate  venture  proves  successful  and  the  region  becomes  thickly 
settled,  the  sub-office  may  be  converted  to  a  main  office  without  loss  of 
equipment. 

The  increase  in  the  cost  of  operating  labor  caused  by  the  division  of  a 
manual  system  is  a  very  serious  obstacle.  In  fact,  while  there  are  con- 
ditions under  which  the  saving  in  the  annual  charges  on  cable  and  conduit 
will  more  than  offset  the  increase  in  operating  labor,  experience  shows 
that  where  the  ultimate  number  of  subscribers  that  may  be  expected  in 
an  office  district  within  fifteen  years  does  not  exceed  the  usual  capacity 
of  a  single  multiple  board  (about  10,000  lines),  and  there  is  no  concen- 
trated group  of  subscribers  at  a  considerable  distance  from  the  best 
location  for  a  single  office,  that  a  one-office  system  will  generally  be  the 
most  economical  when  manual  equipment  is  used.  Furthermore,  in  a 
larger  system  where  division  is  necessary  experience  shows  that  the  saving 
in  the  annual  charges  on  cable  conductors  of  No.  22  B.  &.  S.  gage 
copper  are  generally  not  sufficient  when  compared  with  the  increased 
operating  expense  to  warrant  placing  central  offices  nearer  than  two 
miles  apart. 

Division  of  automatic  systems  may  be  carried  profitably  much  further 
on  account  of  the  very  slow  increase  in  central  office  expenses  resulting 
from  adding  to  the  number  of  offices. 

There  is  still  another  point  to  be  considered  however,  namely,  the 
effect  of  plant  division  on  service.  Therein  lies  a  very  serious  objection 
to  multi-office  manual  systems,  because  slower  service,  more  wrong  con- 
nections and  more  premature  disconnections  are  the  inevitable  results 
of  having  calls  handled  by  two  operators  instead  of  by  one.  There  is 
therefore  strong  opposition,  where  competition  is  keen,  to  dividing  up 
manual  systems,  on  account  of  the  depreciation  of  the  service. 


408 


AUTOMATIC  TELEPHONY 


The  service  in  a  multi-office  automatic  system  is  practically  the  same 
as  in  a  single-office  system.  All  calls  are  trunked  anyhow,  whether  one 
office  is  used  or  more.  The  time  required  for  calling,  the  method  of 
calling,  and  the  number  of  switches  employed  in  setting  up  a  connection 
are  each  the  same  whether  the  system  employs  a  single  office  or  a  number 
of  them. 

Not  only  do  the  reasons  which  make  the  division  of  manual  systems 
undesirable  apply  with  but  little  force  to  automatic  systems,  but  the 
saving  in  cable  and  conduit  realized  by  division  is  much  greater  in  a 
multi-office  automatic  system  for  two  reasons:  First,  because  division 
may  be  carried  much  farther  without  greatly  increasing  operating  ex 


HOLLYWOOD 
057 


HIGHLAND  Q3& 
39 


PROSPECT  0  59 


WLSHIRE 
9il_ 


—  Denotes  Underground 

—  -       Aerial 
O      -        Office 


I— rV 

ADAMS    O7  /DSOUTH 

NORMANDIE  O77 


"M 


VERNON. 


VERMONT    <5  "     v'-~ "v'" 

FlG.  309. — Skeleton   diagram   of   the   automatic   telephone   system   of  Los   Angeles,    Cal. 


penses,  and  second  because  trunks  between  automatic  offices  are  more 
efficient  than  trunks  between  manual  offices  and  a  less  number  is  therefore 
required.  The  greater  efficiency  of  automatic  trunks  is  more  fully 
explained  in  the  chapter  on  traffic. 

Types  of  Automatic  Offices. — There  are  three  types  of  automatic 
switchboard  offices: 

1.  Main  offices. 

2.  Branch  offices. 

3.  Sub-offices. 

A  main  office  is  one  such  as  that  used  in  a  single  office  system.  In  a 
multi-office  system  there  may  be  several  main  offices,  but  each  will  rank 
with  the  others  and  in  no  way  be  subsidiary  to  them. 

A  branch  office  is  subsidiary  to  a  main  office  which  may  have  several 


DEVELOPMENT  ST  I 'DIES  400 

branches  but  acts  as  a  receiving  and  distributing  office  for  the  trunk  calls 
incoming  to  all  of  them  from  other  main  offices  or  the  branches  of  other 
main  offices. 

A  sub-office  is  dependent  upon  a  branch  or  a  main  office  for  its  selectors 
— it  is  made  up  chiefly  of  lineswitches  and  connectors. 

The  Los  Angeles,  Cal.  System. — A  typical  system  showing  the  use  of 
main  and  branch  offices  is  that  in  Los  Angeles,  Cal.  This  system  is 
illustrated  in  Fig.  300.  There  are  six  main  offices,  viz.,  Olive,  West, 
Adams,  South,  Boyle  and  East,  each  with  an  ultimate  capacity  of  10,000 
lines  except  Olive,  which  has  an  ultimate  capacity  of  20,000  lines. 

The  character  of  each  of  the  other  offices  and  the  subscriber's  numbers 
used  in  each  are  shown  in  the  following  list: 

Olive  office  (Main) Numbered  from  10,000  to  19,999,  also  60,000  to  09,999. 

South  office  (Main) Numbered  from  21,000  to  25,999. 

Vermont  Sub-office Numbered  from  20,100  to  26,999. 

Vernon  Branch  office Numbered  from  28,100  to  29,999. 

East  office  (Main) Numbered  from  31,000  to  31,999. 

Highland  Branch  office Numbered  from  38,100  to  39,999. 

Boyle  office  (Main) Numbered  from  41,000  to  41,999. 

West  office  (Main) Numbered  from  51,000  to  55,999. 

Wilshire  Sub-office Numbered  from  56,100  to  56,999. 

Hollywood  Branch  office.  .  .  Numbered  from  57,100  to  57,999. 

Prospect  Branch  office Numbered  from  59,100  to  59,999. 

Adams  office  (Main) Numbered  from  71,000  to  73,999. 

Normandie  Sub-office Numbered  from  77,100  to  77,999. 

South  office  has  one  branch,  Vernon  and  a  sub-office  Vermont; 
West  office  has  two  branches,  Prospect  Park  and  Hollywood  and  a  sub- 
office,  Wilshire;  East  office  has  a  branch,  Highland  Park,  and  Adams  has 
a  sub-office,  Normandie.  The  numbers  in  each  branch  office  necessarily 
commence  with  the  same  digit  as  the  numbers  in  the  main  office  to 
which  it  connects;  that  is,  one  or  more  of  the  sections  of  1000  numbers 
is  taken  from  the  main  office  and  is  set  aside  for  use  in  the  branch.  For 
example:  the  lines  now  equipped  in  South  office  are  numbered  from  21,000 
to  25,099  and  the  numbers  in  its  branch  Vernon,  run  from  28,000  to  29,999. 
It  is,  of  course,  unnecessary  for  a  calling  subscriber  to  know  to  which 
office  he  is  connected  or  to  which  office  the  party  he  desires  to  call  is 
connected. 

The  trunking  between  offices  is  all  automatic.  A  subscriber  for 
instance,  in  the  East  office,  who,  on  the  first  move  of  his  dial  turns  it  from 
the  number  2,  will  automatically  select  a  local  trunk  line  to  a  second 
selector  in  South  office,  and  if  he  makes  the  second  turn  from  the  number 
8,  the  second  selector  at  South  office  will  automatically  connect  him  to  a 
trunk  line  terminating  in  an  idle  third  selector  in  the  "28,000"  group  at 
Vernon  branch  office.  Similarly,  any  call  incoming  to  the  Vernon  branch 
from  other  main-office  districts  must  pass  through  South  office,  because 


410  AUTOMATIC  TELEPHONY 

any  telephone  connected  to  any  other  main  office  will  be  automatically 
trunked  through  to  South  office  as  soon  as  its  dial  is  operated  from  No.  2 
finger  hole. 

Suppose  a  subscriber  connected  to  the  Vernon  office  wishes  to  call 
62,138,  which  is  an  Olive  Street  office  number.  The  first  movement  of  the 
dial  operates  a  first  selector  at  Vernon  office,  and  extends  the  connection 
over  an  idle  trunk  to  a  second  selector  switch  in  the  Olive  Street  office. 
The  second  digit  (2)  will  operate  the  second  selector  at  Olive  Street  office, 
and  extend  the  connection  to  a  third  selector  in  the  "2000"  part  of  the 
Olive  Street  switchboard.  The  third  digit  1  will  extend  the  connection 
to  an  idle  connector  switch  in  the  "100"  group  of  the  "2000"  part. 
The  last  two  digits  will  operate  this  connector  switch  and  complete  the 
connection  to  "38"  in  this  particular  100.  Thus,  it  is  shown  that  out- 
going calls  from  a  branch  office  do  not  go  through  its  main  office  when 
destined  for  another  main  office. 

Suppose,  again,  that  a  Vernon  office  subscriber  is  calling  23,257  which 
is  in  the  South  office.  The  first  movement  of  the  dial  operates  a  first 
selector  in  the  Vernon  office  and  selects  a  trunk  to  a  second  selector  in  the 
Vernon  office.  The  second  movement  of  the  dial  raises  the  shaft  of  this 
second  selector  three  steps,  and  selects  an  idle  trunk  to  a  third  selector  in 
the  "3000"  group  of  the  south  office.  The  third  movement  extends 
the  connection  through  a  local  trunk  in  the  South  office,  to  an  idle  con- 
nector in  the  "200"  group,  and  the  last  two  motions  of  the  dial  result  in 
the  completion  of  the  connection  to  "57"  in  that  particular  hundred. 

It  should  therefore  be  noted,  as  indicated  in  this  paragraph,  that  when 
a  branch-office  subscriber  calls  a  number  in  his  own  main-office  district, 
that  he  does  not  secure  a  trunk  to  his  main  office  by  operating  a  first- 
selector  switch,  because  if  it  should  develop  later,  when  he  made  the 
third  movement  of  his  calling-device  dial,  that  the  desired  number  also 
was  connected  to  the  branch  office  it  would  then  be  necessary  to  extend 
the  connection  back  to  the  branch  office  over  another  trunk.  To  avoid 
this  the  trunks  for  outgoing  calls  from  a  branch  to  its  main  office  termi- 
nate in  second-selector  banks  at  the  branch  and  in  third-selector  switches 
at  its  main  office.  With  this  practice  a  connection  from  one  subscriber 
to  another  in  the  Vernon  branch  office  is  completed  entirely  in  the  branch, 
because  the  trunks  from  the  eighth  and  ninth  levels  of  the  Vernon  second- 
selector  banks  terminate  in  third  selectors  in  the  same  office,  while  the 
trunks  from  the  first  level  lead  to  third  selectors  in  the  "  1000"  group  in 
South  office;  the  trunks  from  the  second  level  lead  to  third  selectors  in 
the  "2000"  group  at  South  office,  etc.,  there  being  an  outgoing  trunk 
group  from  Vernon  to  South  for  each  thousand  section  of  the  South 
office  switchboard. 

Sub-offices. — A  sub-office  installed  by  placing  one  or  more  lineswitch 
units  complete  with  connector  switches  in  a  small  building  at  the  tele- 


DEVELOPMENT  .STUDIES  411 

phonic  center  of  a  district,  generally  1  mile  or  more  distant  from  the 
nearest  central  office.  The  lines  of  all  telephones  in  the  district  are 
brought  to  the  sub-office  and  are  there  connected  to  the  lineswitches. 
The  first  selectors  to  which  these  lineswitches  are  trunked  remain  at  the 
nearest  large  central  office,  consequently  when  a  sub-office  subscriber 
removes  his  receiver  from  his  switch-hook  preparatory  to  making  a  call, 
his  line  switch  instantly  puts  him  into  connection  by  means  of  a  trunk 
with  a  first-selector  switch  at  central  office.  The  connectors  for  handling 
the  calls  to  the  sub-offices  telephones  are  mounted  in  their  usual  places 
on  the  backs  of  the  lineswitch  units,  and  are  connected  by  trunks  to 
the  banks  of  second,  or  third  selectors,  also  located  at  the  nearest  central 
office,  unless  the  sub-office  is  a  comparatively  large  one  (500  lines  or 
over)  in  which  event  the  third  selectors  may  be  installed  at  the  sub- 
office  instead  of  at  the  central  office.  Thus  all  calls  from  and  to  the 
sub-office  are  handled  over  trunks  instead  of  over  subscribers'  lines. 

Since  there  are  usually  but  ten  first  selectors  and  ten  connectors  for 
each  100  lines,  and  since  but  three  pairs  are  needed  for  testing  and  super- 
visory circuits  to  the  sub-office,  a  total  of  23  trunk  pairs  is  sufficient  be- 
tween a  sub-office  of  100  lines  and  the  central  office.  This  leaves  a  net 
saving  of  77  pairs  of  wires  per  100  lines.  In  sub-office  practice  stations 
of  less  than  500  subscribers  are  generally  unattended  and  supervised 
entirely  from  the  central  office  to  which  they  connect.  This  is  so  thor- 
oughly worked  out  (see  Chapter  14)  that  the  wire  chief  can  test  every 
line  entering  each  sub-office  without  leaving  his  desk  at  central.  Sub-office 
of  500  lines  or  more  are  generally  put  into  a  combination  residence  and 
office  building  so  that  one  attendant  living  in  the  building  gives  the  equip- 
ment all  the  attention  that  it  may  require,  although  he  may  spend  much 
of  his  time  elsewhere. 

The  Columbus,  O.  System. — A  typical  system  employing  sub-offices 
is  that  at  Columbus,  Ohio,  which,  as  indicated  in  Fig.  310  employs  one 
large  main  central  office  and  nine  sub-offices  varying  in  size  from  50  to 
625  working  lines. 

To  illustrate  how  connections  between  sub-office  subscribers  are 
made,  some  imaginary  calls  will  be  followed  through  the  Columbus 
system. 

Suppose  that  14,625  connected  to  the  Blake  office  in  the  extreme 
northern  part  of  the  city  is  calling  13,578,  who  is  connected  to  the  Han- 
ford  office  in  the  southern  part  of  the  city.  When  the  Blake  subscriber 
turns  his  dial  from  finger  hole  1  his  lineswitch  plunges  into  its  bank  and 
connects  his  line  to  an  idle  trunk  terminating  in  a  first-selector  switch  at 
the  main  office.  This  first  selector  is  operated  as  the  dial  rotates  back 
to  normal  and  extends  the  connection  to  an  idle  second  selector,  also  in 
the  main  office.  When  the  calling  party's  dial  is  operated  from  finger 
hole  3  this  second  selector  extends  the  connection  to  a  third  selector  in 


412 


A  U  TOM  A  TIC  TELEPHON  Y 


the  "13,000"  group  of  switches  at  Main.  The  next  turning  of  the  dial 
from  finger  hole  5  results  in  the  operation  of  this  third  selector,  which 
extends  the  connection  to  an  idle  trunk  terminating  in  a  connector  switch 


BLAKE  .OFFICE  ~| 


SHEPARO 

OFFICE 

( Proposed)     i 


Or 

HILLTOP 
OFFICE  (.Proposed) 

City  Limits  (Approximate) 

'•A 


HANFORD 
OFFICE 
196  Lines 
I 


Fig.  310. — Skeleton  diagram  of  automatic  telephone  system  of  one  main  central   office 
surrounded  by  sub-offices  in  Columbus,  O. 

on  the  "500"  board  at  the  Hanford  station.     This  connector  switch 
responds  to  the  last  two  motions  of  the  dial  and  completes  the  connection 


O 


6LAKE  DISTRICT 
&TA. 

.■Line  Switch 


Calling 
Telephone,  I4d25 


Connector  Switch 


MAIN    OFFICE 


HANFORD  DIST. 
STA. 

Called 


O 


7elephone,l3578 

Fig.  311. — Diagram  illustrating  steps  in  setting  up  a  connection    from  a  subscriber  con- 
nected to  Blake  to  one  connected  to  Hanford  sub-office. 


to  "78"  in  that  group,  thus  ending  the  operation  of  connecting  the  Blake 
subscriber  to  Hanford  subscriber  13,578.  This  connection  is  illustrated 
diagrammatically  in  Fig.  311. 


DEVELOPMENT  STUDIES 


413 


Trunks  for  Reverting  Sub-office  Calls.— Since,  as  stated,  a  sub-office 
line  is  instantly  connected  to  a  trunk  to  main  office  as  soon  as  a  calling; 
subscriber  lifts  his  receiver  from  his  switch  hook  (or  makes  the  first  turn 
of  his  dial  if  a  three-wire  system  is  used),  it  is  evident  that  even  if  he 
should  wish  to  call  another  subscriber  connected  to  his  own  office  that  the 
connection  will  pass  through  the  main  office  and  will  then  be  extended 
back  to  the  sub-office,  in  which  the  call  originated,  when  the  connector 
switch  trunk  is  occupied.  Therefore,  as  indicated  in  Fig.  312,  such  a 
connection  occupies  two  trunks  between  the  sub-office  and  its  main  office. 

Where  conditions  warrant  the  use  of  somewhat  more  complicated 
and  expensive  sub-office  apparatus,  equipment  may  be  installed  at  a 
sub-office  which  will  avoid  the  necessity  of  using  any  trunk  between 
a  sub-office  and  its  main  office  during  a  local  conversation.     When 

Called  Telephone 


Connector 
Switch 


Line  Switch 
DISTRICT  STA. 


MAIN  OFFICE 


o 


Calling 
Telephone 

Fig.  312. — Showing  switches  and  inter-office  trunks  used  for  a  reverting   sub-office. 


this  apparatus  is  used  the  trunk  from  the  sub-office  to  the  main  office  is 
occupied  until  by  the  selection  of  the  desired  "  1000"  or  "  100"  group  the 
calling  subscriber  indicates  that  the  connection  is  to  revert  to  his  own 
sub-office,  whereupon  the  entire  connection  is  automatically  released  and 
a  local  connection  made  to  an  idle  third  selector  or  connector  in  the 
sub-office. 

Such  apparatus  is  especially  adapted  to  stations  having  a  preponder- 
ance of  the  following  characteristics: 

1.  A  comparatively  large  percentage  of  local  calls. 

2.  Comparatively  expensive  trunks  to  main  office. 

3.  Comparatively  constant  local  supervision,  such  as  generally  obtains 
in  a  sub-office  of  more  than  500  lines. 

While  this  apparatus  is  entirely  practicable  it  should  only  be  installed 
where  a  study  of  local  conditions  leads  to  the  conclusion  that  it  is  war- 


414  AUTOMATIC  TELEPHONY 

ranted.  Therefore,  to  simplifj'  the  following  discussion  no  further 
reference  will  be  made  to  it. 

Utility  Trunks  to  Sub-offices. — To  the  talking  trunks  for  each  sub- 
office  must  be  added  a  few  utility  trunks.  These  trunks  are  no  more 
"incoming"  than  they  are  "outgoing,"  but  it  is  immaterial  which  they 
are  called  as  they  can  be  grouped  with  any  of  the  other  trunks.  They  are 
used  for  furnishing  ringing  and  busy-signal  currents,  to  the  sub-office  from 
its  main  office  and  for  supplying  the  main  office  attendants  with  super- 
visory signals,  which  enable  them  to  supervise  the  sub-office  from  main. 
The  wire  chief  at  main  also  uses  some  of  these  utility  wires  for  operating  a 
"test  distributor"  at  each  sub-office  which  enables  him  to  connect  to  a 
test  connector  on  any  lineswitch  board  and  thus  test  all  sub-office  lines 
from  the  main  office.  Five  utility  trunks  are  generally  specified  for  the 
smaller  offices  and  three  for  the  larger.  The  reason  for  this  is  that  sub- 
offices  of  over  500  lines  are  generally  equipped  with  ringing  and  busy- 
signalling  machines  of  their  own  so  that  trunks  from  the  main  office 
are  not  required  for  these  purposes. 

Relative  Advantages  of  Sub-offices  and  Branch  Offices  or  Small 
Main  Offices. — Generally,  a  branch  office  or  a  small  main  office  is  best 
suited  to  a  district  in  which  there  is  a  large  "community-of-interest," 
also  where  enough  equipment  is  to  be  installed  to  warrant  the  expense  of 
a  constant  attendant,  at  least  during  the  hours  from  6  a.m.  to  10  p.m. 

A  sub-office  is  best  suited  to  a  territory  where  the  community-of- 
interest  is  comparatively  small  and  where  not  more  than  1000  lines  are 
to  be  served.  It  is  also  especially  suited  to  an  isolated  district  which 
would  require  a  heavy  investment  in  cable  to  connect  it  to  the  nearest 
office  and  where  the  expense  of  a  constant  local  attendant  would  be 
unwarranted.  A  sub-office  uses  the  simplest  known  automatic  apparatus 
and  all  calls  to  or  from  it  can  be  supervised  at  its  main  office. 

A  sub-office  may  often  be  installed  as  an  excellent  expedient  for 
relieving  a  district  in  which  all  cable  pairs  have  been  exhausted.  To 
accomplish  this  the  present  line  cable  may  be  converted  into  a  trunk  cable, 
so  that  a  100-pair  line  cable,  for  example,  may  be  made  to  carry  all  trunks 
to  a  400-line  district  station. 

A  strong  objection  to  a  small  main  or  a  branch  office,  as  compared 
with  a  sub-office  is  that  it  requires  its  own  individual  group  of  outgoing 
trunks  to  every  main  office  in  the  system.  Trunking  studies  which  have 
been  made  indicate  that  in  systems  with  an  ultimate  capacity  of  less  than 
10,000  lines  the  total  trunk  mileage  is  generally  the  least  where  only 
sub-offices  are  employed,  with  the  exception  that  a  second  main  office  is 
sometimes  found  advisable  in  a  comparatively  isolated  suburb,  with  a 
business  center  of  its  own,  or  in  a  district  requiring  a  comparatively  large 
office;  i.e.,  one  of  more  than  100  lines. 


DEVELOPMENT  STUDIES 


415 


Ultimate  Capacities. — When  an  area  is  likely  to  grow  beyond  10,000 
subscribers'  stations  within  a  15-year  period,  and  especially  if  more  than 
one  main  office  or  if  a  branch  office  will  be  required  it  generally  would  be 
unwise  to  attempt  to  serve  it  with  four  digit  numbers.  Instead,  five 
figure  numbers  should  be  used;  i.e.,  the  system  should  have  an  ultimate 
capacity  of  100,000  lines  rather  than  10,000.  There  would  be  two 
disadvantages  to  the  100,000  line  capacity  system. 

1 .  Third  selectors  would  be  added,  increasing  the  first  cost  and  slightly 
increasing  operating  expenses. 

2.  Five  motions  of  the  dial  instead  of  four  would  be  required  to  call 
each  number. 


300  Lines 
A  {\No's.fyOO-ie$99 


900  Lines 

No's.ZIJOO- 

B  )  3U99 


200  Lines 
Nos.ld&OO- 
18,799 

o 

c 


Fig.  313. — Skeleton  diagram  of  a  system  of  three  main  offices,  "Main"  B,  and  D,  and 
three  district  stations  A,  C,  and  E. 


Neither  of  these  are  very  serious  disadvantages,  and  are  outbalanced 
by  the  two  advantages  which  would  be: 

1.  Room  in  the  numbering  system  for  a  large  growth  in  all  offices. 

2.  A  reduction  in  the  trunk  mileage. 

In  explanation  of  the  second  advantage,  reference  is  made  to  Fig. 
313,  which  shows  a  hypothetical  system,  which  if  of  10,000  capacity, 
would  require  six  groups  of  trunks  from  each  outlying  main  office  to 
central  office,  but  if  changed  to  one  of  100,000  capacity  with  the  number- 
ing arranged  approximately  as  shown  in  Fig.  313,  would  require  but  one 
group  of  trunks  for  calls  from  each  outlying  main  office  to  the  central 
main  office.  B  and  D  are  treated  as  main  offices  because  it  is  probable 
that  they  would  require  the  least  trunk  mileage  when  so  arranged. 


410  AUTOMATIC  TELEPHONY 

This  being  the  case,  the  size  and  relative  importance  of  each  make  its 
treatment  as  a  main  advisable,  so  that  an  accident  to  its  trunk  cables  will 
not  throw  it  entirely  out  of  service. 

A,  C  and  E  are  shown  as  sub-offices  and,  therefore,  their  line  numbers 
commence  with  the  same  digit  that  those  in  the  central  main  office  do, 
i.e.,  with  1. 

Main  Office  Locations. — The  proper  theoretical  location  for  a  main 
office  for  a  given  district  is  the  point  to  which  all  telephones  may  be  con- 
nected with  the  smallest  total  wire  mileage.  This  point  is  at  the  intersec- 
tion of  two  lines  at  right  angles  to  each  other  and  each  of  which  divides 
the  subscribers'  lines  into  two  groups  equal  in  number.  When  two  or 
more  main  offices  are  used,  each  should  be  at  the  telephonic  center  of  its 
district  and  the  boundary  line  between  two  adjacent  offices  should  be 
equidistant  from  each  and  perpendicular  to  a  line  joining  the  two  offices 
which  it  separates. 

Location  of  a  Single  Central  Office. — When  the  15-year  map  does  not 
show  a  concentrated  group  of  subscribers  at  a  distance  of  1  mile  or  more 
from  the  telephonic  center  of  the  entire  city  it  may  be  taken  for  granted 
that  a  single  office  is  the  most  economical  arrangement.  When  the 
theoretical  site  for  the  office  has  been  determined  it  may  be  advisable  to 
depart  from  it  somewhat  in  order  to  use  existing  conduit  leads  or  an 
existing  building  or  to  secure  new  property  at  a  more  reasonable  price. 
An  engineering  study  will  be  necessary  to  decide  how  far  the  office  may 
be  moved  economically  in  order  to  allow  for  any  of  these  factors. 

Central  Office  Locations  in  a  Multi-office  Automatic  System. — To 
take  up  the  matter  in  its  simplest  form  first,  a  system  in  which  there  is 
but  one  main  central  office,  together  with  a  number  of  sub-office, 
will  be  given  primary  consideration. 

The  location  of  this  central  office  will  be  affected  by  the  number  of 
sub-offices  installed,  unless  the  same  number  of  subscribers  should  per- 
chance be  put  into  sub-offices  on  each  of  the  four  sides  of  the  office;  but 
take,  for  example,  a  case  in  which  the  business  houses  are  concentrated 
at  one  side  of  the  city  on  account  of  a  lake,  river  or  other  obstruction. 
Then  the  number  of  sub-offices  installed  in  the  residence  section,  directly 
opposite  the  obstruction  will  move  the  telephonic  center  toward  the 
obstruction  and  toward  the  actual  center  of  the  business  district.  It  is, 
therefore,  necessary  in  order  to  locate  the  central  office  properly  to  arrive 
at  a  fairly  accurate  idea  as  to  which  subscribers  will  be  connected  into 
sub-offices,  and,  consequently,  trunk  their  calls  into  the  main  office. 

Sub-office  Locations. — Installing  lineswitches  for  serving  a  given 
group  of  subscribers  in  a  sub-office  instead  of  in  the  central  office  affects 
the  following  items  of  first  cost : 

1.  Increases  cost  of  switchboard  and  power  equipment. 

2.  Increases  or  decreases  cost  of  buildings  and  lots. 


DEVELOPMENT  STUDIES  417 

3.  Decreases  cost  of  cable  and  conduit. 

A  careful  study  should  be  made  of  each  proposed  location  for  a  sub- 
office  to  determine  the  effect  upon  each  of  these  items.  After  these 
results  have  been  determined,  the  effect  upon  annual  charges  and  operat- 
ing expenses  should  be  calculated. 

Effect  on  the  Cost  of  Switchboard  and  Power  Equipment. — The  cost  of 
switchboard  and  power  equipment  for  serving  a  given  number  of  sub- 
scribers from  a  sub-office  is  greater  than  that  of  the  equipment  necessary 
to  serve  the  same  number  of  subscribers  from  the  central  office,  because 
while  the  same  type  of  equipment  is  used,  the  cost  of  the  trunking,  power 
and  supervisory  apparatus  is  increased. 

Each  trunk  from  the  main  office  to  a  sub-office  must  be  provided,  in 
addition  to  the  usual  second  or  third  selector,  with  cross-connecting 
frame  terminals  at  each  end,  with  a  repeater  at  the  main  office  end  and 
with  certain  test  and  supervisory  equipment.  Each  trunk  from  the  sub- 
office  to  the  main  office  requires,  in  addition  to  the  usual  first  selector, 
cross  connecting  frame  terminals  at  each  end,  and  a  repeater  at  the  sub- 
office.  When  secondary  lineswitches  are  used  between  the  lineswitches 
and  first  selectors  at  the  central  office  the  number  of  first  selectors  is 
increased  by  the  use  of  sub-offices.  For  example,  when  secondary  line- 
switches  are  used  the  number  of  first  selectors  required  to  handle  calls 
for  5000  lines  all  connected  to  one  central  office  would  be  less  than  if  the 
same  system  were  built  with  3000  lines  connected  direct  to  the  central 
office,  and  the  remaining  2000  lines  connected  to  four  different  sub-offices 
of  500  lines  each. 

The  cost  of  the  power  equipment  is  greater  in  a  multi-office  system 
than  in  a  single-office  system,  because,  as  is  readily  apparent,  it  is  more 
expensive  to  install  apparatus  for  furnishing  the  necessary  amount  of 
power  from  a  number  of  small  units  than  from  one  large  plant. 

Generally  in  sub-offices  of  from  100  to  500  lines,  the  battery  charging 
will  be  done  from  the  main  office  and  in  sub-offices  of  500  lines  and  up,  the 
charging  and  ringing  equipment  will  be  located  at  the  sub-office. 

In  sub-offices  of  500  lines  or  over,  and  especially  when  situated  at  a 
considerable  distance  from  the  main  office,  it  is  generally  advisable  to  use 
secondary  lineswitches  on  the  trunks  in  order  to  reduce  the  cost  of  cable 
pairs,  but  that  should  be  a  matter  of  special  study  for  each  sub-office. 

Effect  on  Cost  of  Central  Office  Building  and  Lots. — The  total  number 
of  cubic  feet  of  space  required  for  switchboard  and  power  equipment  is 
practically  unaffected  by  the  use  of  sub-offices,  because  it  is  customary  in 
sub-offices  to  make  aisles  smaller,  ceilings  lower,  and  generally  to  econo- 
mize in  space  more  than  in  the  main  central  offices,  so  that,  although  a 
little  more  equipment  is  to  be  taken  care  of  when  sub-offices  are  used, 
experience  shows  that  the  total  number  of  cubic  feet  of  space  required  is 
about  the  same.     The  cost  of  the  space  in  a  main  office  fire-proof  building, 

27 


418  AUTOMATIC  TELEPHONY 

may  be  taken  at  the  rate  of  19  c.  per  cubic  foot,  (1914  prices)  for  purposes 
of  comparison;  while  experience  would  indicate  that  in  sub-offices  the  cost 
per  cubic  foot  of  space  may  be  taken  at  not  to  exceed  10  c,  because  the 
buildings  are  of  rough  finish,  and  not  provided  with  plumbing  or  elaborate 
heating  systems.  Since  the  total  number  of  cubic  feet  of  space  required 
in  a  sub-office  system  is  practically  the  same  as  in  a  one-office  system, 
nad  the  cost  of  the  space  in  sub-offices  is  cheaper  than  in  the  main  office, 
it  follows  that  the  total  cost  of  all  office  buildings  may  be  reduced  by  the 
use  of  sub-offices. 

It  will  generally  be  found  that  the  land  on  which  a  sub-office  building 
may  be  installed  is  decidedly  cheaper  than  that  on  which  the  main  central 
office  building  is  placed.  It  is  suggested  that  when  an  operating  company 
purchases  a  lot,  puts  up  a  small  sub-office  on  the  rear  of  it,  and  then  sells 
the  rest  for  residence  purposes  that  the  cost  of  the  land  occupied  by  the 
sub-office  building  may  generally  be  reduced  to  a  very  nominal  figure. 
Some  operating  companies  have  made  the  cost  of  building  space  and  lot 
that  can  properly  be  charged  against  the  sub-office,  a  comparatively  small 
figure  by  purchasing,  or  renting  a  residence,  installing  the  sub-office 
equipment  in  one  room  of  it  and  then  renting  the  rest  of  it  to  an  employe 
of  the  company.  A  point  to  be  considered  in  this  connection  is  that  the 
heating  of  the  substation  equipment  during  clamp,  rainy  weather  or  the 
cold  winter  months  becomes  a  very  simple  matter  when  it  occupies  one 
room  of  a  residence. 

No  general  rules  can  be  laid  down  for  determining  whether  or  not  the 
total  cost  of  office  buildings  and  lots  will  be  increased  or  decreased  by  the 
use  of  sub-offices.  The  necessary  data  must  be  secured  by  a  study  of 
local  conditions.  Generally  if  the  use  of  sub-offices  is  decided  upon 
before  an  unecessarily  large  central  office  building  is  erected,  the  tendency 
will  be  to  lessen  rather  than  to  increase  the  total  real  estate  investment. 

Annual  Charges  and  Operating  Expenses. — While  the  studjr  of  the 
first  cost  of  each  office  arrangement  as  compared  with  the  first  cost  of 
other  suggested  office  arrangements  is  necessary  and  interesting,  it 
should  only  be  a  step  in  making  up  a  comparison  of  the  annual  charges 
and  operating  expenses  of  the  different  arrangements. 

Annual  Charges  on  Central  Office  Equipment. — At  an  average  figure, 
taxes  on  central  office  equipment  may  be  taken  at  the  rate  of  1^2  per 
cent  per  annum,  and  interest  at  the  rate  of  6  to  8  per  cent  per  annum. 
Depreciation  and  obsolescence  on  automatic  equipment  should  be  calcu- 
lated on  a  life  of  not  less  than  15  years  and  it  would  not  be  unreasonable 
to  consider  the  life  20  years.  The  amount  which  must  be  set  aside 
annually  at  6  per  cent  compound  interest  to  equal  100  per  cent  in  15 
years  is  4.3  per  cent  of  first  cost;  therefore,  this  percentage  may  be  used 
in  calculating  depreciation  and  obsolescence,  although  the  life  of  auto- 
matic  apparatus   is  full   20  years.     At  8  per  cent   the  annual  charge 


DEVELOPMENT  STUDIES  419 

is  3.05  per  cent.  The  actual  cost  of  maintenance  material,  or  renewals, 
for  automatic  central  office  equipment,  it  has  been  found  from  experience, 
will  be  covered  by  a  charge  of  3^  per  cent  per  annum.  Insurance  in 
fire-proof  buildings  is  taken  at  1  per  cent  per  annum.  Adding  together 
the  percentages  indicated  for  taxes,  interest,  depreciation,  maintenance 
and  insurance,  makes  a  total  of  13.3  per  cent.  Or  14  per  cent  at  the 
higher  rates. 

When  estimating  the  annual  charges  on  sub-office  equipment,  the 
figures  might  be  taken  at  14.3  per  cent  instead  of  13.3  per  cent — the  extra 
1  per  cent  being  added  to  cover  any  additional  cost  of  furnishing  power 
and  heat  for  the  equipment  at  the  sub-office. 

Central  Office  Operating  and  Maintenance  Labor. — The  average 
operating  and  maintenance  labor  cost  in  branch  offices  or  sub-offices 
as  small  as  500  lines  is  not  more  than  10  per  cent  to  20  per  cent  greater 
than  in  the  larger  offices.  For  offices  below  that  size  there  is  usually  a 
greater  increase,  although  this  depends  considerably  on  local  conditions. 
Our  investigations  lead  us  to  believe  that  the  annual  labor  cost  allotted 
to  the  average  sub-office  of  the  smallest  size — that  is,  100  lines — 
will  rarely  be  increased  more  than  25  per  cent  over  what  would  be  the 
operating  and  labor  maintenance  cost  for  the  same  amount  of  equipment 
installed  in  a  central  office. 

As  already  mentioned  the  wire  chief  in  a  city  using  sub-offices 
can  test  all  lines  from  his  desk  at  central  without  the  aid  of  an  assistant 
at  the  sub-office.  All  trouble  and  information  calls  come  to  central 
office  just  as  in  a  one-office  system,  and  all  records,  supervision  and 
management  remain  centralized  at  the  main  office  so  that  the  only  labor 
cost  affected  is  that  necessary  to  take  care  of  the  switchboard,  power  and 
cross-connecting  frame  apparatus. 

Annual  Charges  on  Lines  and  Trunks. — The  annual  charge  per 
working  pair  mile  for  subscribers'  circuits  in  No.  22  gage  cable  is  com- 
monly taken  at  $2.55.  In  the  case  of  a  snaller  office,  say  one  with  2000 
lines  or  less  the  charge  should  be  somewhat  higher^  about  $2.65. 

The  higher  charge  is  due  to  the  smaller  number  of  cables,  and  conse- 
quently, greater  loss  of  efficiency  in  the  smaller  office.  In  No.  19  gage 
cable,  the  charge  per  working  pair  per  mile  is  generally  taken  at  $4.25.  The 
annual  charges  on  local  trunk  circuits  are  generally  taken  to  be  somewhat 
less  than  on  the  subscribers'  lines.  The  charge  per  working  pair  mile  on 
No.  22  gage  circuits  should  be  taken  at  about  $2.40  per  annum,  and  on 
No.  19  gage  cable  at  $3.60  per  annum. 

If  more  accurate  figures  than  these  are  desired,  the  cost  of  the  conduit 
manholes  and  underground  cables,  poles  with  wire,  cable  and  suspension 
may  be  carefully  computed,  and  the  annual  charges  calculated  from  the 
following  figures: 


420 


A  U  TOM  A  TIC  TV  LEI 'HON  Y 


Annual  charge  on  underground  conduit  and  manholes. . .  9  per  cent 

Annual  charge  on  underground  cable 11^  per  cent 

Annual  charge  on  pole  lines 213-2  per  cent 

Annual  charge  on  aerial  cable  and  suspension 143-2  Por  cent 

Additional  Main  Offices. — In  a  very  large  city  system  covering  many 
square  miles  and  including  not  only  a  number  of  suburbs  or  former  suburbs 
with  their  own  business  centers,  but  also  business  districts  which  have  a 
very  large  number  of  intercommunications,  more  than  one  main  office 
will  be  very  clearly  indicated.     As  a  rule  the  best  way  to  lay  out  such  a 


A-7  Q 1000 Lines 


A-6 


500  Lines 


C-6Q 


1000  Lines 


$000  Lines 


Fig.  314, 


500  Lines 
-Diagram  representing  hypothetical  system  of  4  main  offices  A,  B,  C,  and    D 
and  4  subsidiary  offices,  A-6,  A-7,  C-6,  and  C-7. 


city  is  to  make  studies  with  the  main  offices  at  the  points  which  appear 
to  be  the  most  likely  locations  for  them,  and  then  as  far  as  possible  to 
take  care  of  the  rest  of  the  territory  with  sub-offices  surrounding  each 
main  office  and  connected  with  it. 

While  it  is  generally  advisable  to  make  a  separate  engineering  study 
for  a  subsidiary  office,  which  is  expected  to  have  an  ultimate  capacity  of 
1000  lines  or  more,  it  will  be  found  that  the  trunking  plan  becomes  more 
complicated  and  the  trunk  mileage  is  likely  to  be  greater  in  a  large  system 
where  branch  offices  are  used,  than  it  is  where  sub-offices  are  used.  For 
illustration,  refer  to  Fig.  314  which  outlines  a  hypothetical  system  in 
which  there  are  four  main  offices  A,  B,  C,  and  D,  each  serving  5000  lines. 
C  has  two  subsidiary  offices  C-6  and  C-7  serving  1000  and  500    lines 


DEVELOPMENT  STUDIES 


421 


respectively.     A  has  two  subsidiary  offices  A-6  and  A-l  serving  500  and 
1000  lines  respectively. 

Suppose  the  subsidiary  offices  to  be  branch  offices;  then  C-6,  for 
example,  will  require  nine  groups  of  outgoing  trunks,  five  to  "C"  office 
and  one  group  to  each  of  the  other  offices,  except  A -6  and  A-l.  Suppose, 
also,  that  the  number  of  busy-hour  calls  is  two  per  line.  The  total 
number  of  outgoing  busy-hour  calls  from  each  of  the  offices  to  any  other 

C(  S  X  7?^ 
office  may  be  calculated  by  the  well  known  formula  — - — ^ — -    =    out- 
going calls.     In  this  formula  S  represents  the  number  of  rush  hour  calls 
originating  in  the  sending  office,  R  the  number  originating  in  the  receiv- 
ing office,  T  the  number  of  busy-hour  calls  in  the  entire  system,  and  C 


no 

100 
90 

£80 

o 
<r 

£70 

Q. 

*60 

z 

D 

a 

!l50 

o 

£40 

s 

3 

z30 
20 
10 


0      200    4.00    600    800   1000120014001600180020002200240026002800300032003400360038004000 
BUSY  HOUR  CALL  CARRIED  PER  TRUNK  GROUP 

Fig.  315. — Curve  showing  carrying  capacities  of  trunk  groups  of  hypothetical  system  in 

Fig.  314. 


is  a  corrective  factor  proportional  to  the  "  community-of -interest"  at  the 
sending  office.  Suppose  that  the  community-of-interest  factor  C  for 
each  of  the  branch  offices  is  1  when  its  own  main  office  is  being  con- 
sidered, and  is  0.75  when  one  of  the  other  main  offices  is  being  consid- 
ered. Suppose  that  the  holding  time  of  connections  in  this  system  to  be 
such  that  the  trunk-carrying  capacities  are  represented  by  the  curve  in 
Fig.  315.  Calculations  with  these  hypotheses  give  the  results  for  the 
calls  and  trunks  from  and  to  C-6  shown  in  the  following  tables. 

The  trunks  are  figured  for  a  system  without  secondary  lineswitches 
and  in  which  the  largest  group  of  trunks  is  ten.  It  is  noted  that  the 
total  trunk  mileage  required  for  handling  the  incoming  and  outgoing 
traffic  of  C-6,  when  it  is  considered  as  a  branch  office  is  234  plus  78  or 
312. 


422 


AUTOMATIC  TELEPHONY 
Branch  Office  Arrangement 


(Outgoing  from  C-6) 


(Incoming  to  C-6) 


To  office 

Busy    hour 
call 

Trunks 
required 

Trunk 
mileage 

1 

From 
office 

Busy  hour 
calls 

Trunks 
required 

Trunk 
mileage 

c 

435 

30 

30 

c 

1511 

70 

70 

C-7 

44 

4 

8 

C-7 

44 

4 

8 

A 

424 

20 

60 

B 

326 

17 

85 

D 

326 

17 

51 

Total 

1555 

88 

234 



1555 

74 

78 

Now  consider  C-6  as  a  sub-office  containing  lineswitches,  connectors 
and  third  selectors  only.  In  this  event,  all  of  its  trunks  will  terminate  in 
its  main  office  C,  in  fact  all  of  its  connections  will  be  made  through  C. 
Without  secondary  lineswitches  it  will  have  ten  groups  of  outgoing 
trunks,  one  group  for  each  lineswitch  unit  of  100  switches;  and  its  incom- 
ing trunks  will  be  divided  into  as  many  groups  of  ten  as  may  be  necessary 
to  carry  the  traffic.  No  other  factors  are  to  be  taken  into  account 
because  third  selectors  are  installed  at  C-6. 

While  all  outgoing  trunks  from  C-6  terminate  in  C,  its  traffic  will 
make  necessary  more  outgoing  trunks  from  C  to  other  offices  than  would 
be  necessary  if  it  did  not  exist  and  these  must  be  taken  into  account  in  a 
comparison.  The  following  table  shows  the  trunks  required  from  C-6  to 
C,  aso  the  additional  trunks  from  C  to  other  offices. 


Sub-office  Arrangement 
Trunks  Into  and  Out  of  C-6 


To  Office 


Busy 
hour 
calls 


Trunks 
required 


Trunk 
mileage 


From 
office 


Busy 
hour 
calls 


Trunks 
required 


Trunk 
mileage 


c 

2000 

100 

100 

C 

2000 

90 

90 

c 

For  Su- 
pervi- 
sion, etc. 

3 

3 

Additional  Outgoing 

Trunks  Required  from  C  to  Other  Offices  on  Account  of  C-6 

C-7 
A 
B 
D 

44 
424 
326 
326 

4 
19 
16 
16 

4 
38 
64 
32 

Total 

241 

90 



DEVELOPMENT  STUDIES  423 

Comparing  this  table  with  the  previous  one  we  note  that  the  total 
trunk  mileage  is  331  as  against  312,  so  that  the  branch  office  is  the  most 
efficient  arrangement  by  19  trunk  miles.  The  introduction  of  secondary 
lineswitches  on  the  trunks  from  the  sub-office  C-6  to  C  would  reverse  the 
conditions,  however.  Such  switches  would  reduce  these  trunks  to 
seventy  and  thus  make  a  saving  of  30  trunk  miles  and  a  total  saving  of 
11  miles  as  compared  with  the  branch  office  plan.  There  would  be 
practically  no  economy  in  the  use  of  secondary  lineswitches  on  the  out- 
going trunks  from  C-7  if  it  were  a  branch  office  because  the  trunks  would 
be  divided  into  so  many  small  groups. 

A  further  saving  can  be  made  with  the  sub-office  arrangement  by  using 
secondary  lineswitches  on  the  outgoing  trunks  from  C  to  other  main 
offices.  All  out  traffic,  not  only  from  C  but  also  from  C-7  and  C-6  to  the 
other  main  offices,  would  pass  through  these  secondaries;  but  with  the 
branch  office  arrangement  where  the  outgoing  traffic  is  handled  by  the 
small  groups  of  trunks  terminating  direct  in  C-6  there  would  be  very  little 
saving  in  C-6's  trunks  by  the  use  of  such  switches. 

The  sub-office  plan  will  appear  more  favorably  still  in  any  plant 
where  long-distance  connections  are  afforded  to  the  subscribers  and  where 
a  subscriber  places  his  order  for  such  a  connection  by  the  common  method 
of  calling  "0"  and  thus  securing  an  idle  trunk  to  the  recording  operator. 
These  trunks  would  be  separate  and  distinct  from  a  branch  office's  first 
selector  banks  direct  to  the  toll  board,  but  would  not  be  taken  into 
account  in  arranging  for  the  trunking  facilities  of  a  sub-office,  whose 
first  selectors  are  in  its  main  office.  In  many  exchanges  a  one-figure 
number  is  used  for  some  other  purpose  besides  calling  the  long-distance 
board.  Wherever  this  practice  occurs  it  requires  a  separate  trunk  group 
from  any  branch  office  that  may  be  installed,  but  does  not  affect  the 
trunks  from  a  sub-office.  Of  course  a  separate  engineering  study  should 
generally  be  made  of  each  case  to  determine  whether  a  branch  office  or  a 
sub-office  is  most  practical,  but  the  foregoing  illustrates  in  a  general  way 
the  factors  that  would  effect  the  result,  and  as  a  general  rule  a  branch 
office,  or  even  a  second  main  office  should  not  be  installed  unless  its 
advantage  as  against  the  use  of  a  sub-office  is  very  clearly  indicated. 

By  employing  sub-offices  only  the  trunking  system  is  kept  as  simple 
as  possible,  regardless  of  the  number  of  offices  used. 


INDEX 


Advancing  slip,  125 
Air  conditioning,  357 
Alarm  fuse,  357 

fuses  and  voltage,  357 

scheme,  release  signal  and  tell-tale, 
360 
American  Automatic  Telephone  Co.,  3 

35,  132,  227 
American  Telephone  &  Telegraph  Co. 

389 
Annual  charges,  418 
Apartment    house    automatic   exchange 

184 
Automanual,  36 

system,  253 
Automatic  Telephone  Mfg.  Co.,  35 
Automatic  order  wire  system,  208,  211 

telephone  instrument,  38 

telephony  defined,  1 

through  switching,  308 

toll  lines,  297 

traffic  distributor,  204,  216 

B 

Back  bridge  relay,  57,  70 

Baer,  Fred.  L.,  403 

Bank  of  switch,  W.  E.  Co.,  242 

slip,  124 

terminal  boards,  112 

wiring,  5,  55 
Battery  distribution  scheme,  351 
Bay,  of  switches,  113 
Booster  telephone  circuit,  153 
Bridge  cut  off,  47 
Brush  rods,  panel  system,  246 
Brushes,  or  wipers,  panel  system,  243 
Burns,  A.  E.,  308 
Busy  test  relay,  57 

tone  distribution,  354 


Call  indicator,  panel  system,  251 
indicator,  W.  E.  Co.,  240 
indicators,  341 


Calling  device,  38 

speed  indicator,  364 
Cam  of  calling  device,  39 
springs,  63,  70 
springs,  11-R,  106 
Campbell,  W.  Lee,  402,  406 
Central  office  meters,  193 
Chain  relay,  70,  87 
Charging  machines,  345 
Choke  relay,  88 
Christensen,  P.  V.,  207,  403 

electropneumatic  selector,  207 
Clement  Auto-manual,  36 

Edward  E.,  253 
Clutch,  panel  system,  246 
Code  call  system,  170 

ringing,  311 

ringing  machine,  312 
Coin  box,  199 

collectors,  199 
Columbus,  Ohio,  system,  411 

toll  calling,  308 
Comb,  45 

Combined  levels  and  trunks,  146 
Community    automatic    exchange,    313, 

317,  319 
Commutator,  W.  E.  Co.,  242 
Compagnie  Francaise  Thomson-Houston, 

35 
Complaint  desk,  373 

trunks,  379 
Composite  dialing,  304 
Condenser  for  spark,  50,  52,  61 
Conference  arrangement,  168 
Connecting  links,  10 
Connector  circuit,  266 

combined  toll  and  local,  294 

duties,  5,  57 

one-digit  group,  99,  102 

trunk  shelf,  111 

two-digit  group,  99 

supervision,  61 

switch,  3,  53,  256 

switch  circuit,  57 

rotary  (see  "group") 
Connolly  &  McTighe,  1 
425 


426 


INDEX 


Connolly,  semi-automatic,  253 
Cost  of  real  estate,  417 
of  switchboard,  417 
Counter  E.M.F.  cells,  345,  349 
Credit  system,  175 
Cumulative  time  meter,  198 
Current  reversal,  60 
Cut-overs  to  automatic,  325 


1) 


Dead  number  trunks,  376 
Deakin,  Gerald,  185 
Dean  Electric  Co.,  231 
Definitions  of  traffic,  381 
Development  studies,  404 
Dial  of  calling  device,  39 

tone,  63 

trunk  tone,  353,  354 
Dictation  service,  176 
Directing  relay,  96 
Discriminating  tone,  278 
District  selector,  W.  E.  Co.,  238,  241 
Districts,  27 

Divided  bank  wiring,  123 
Double  number  pick-up,  167 


E 


Elevator,  W.  E.  Co.,  242 
11-R  springs,  63 
Emergency  service,  174 
Equating  factors,  operators,  336 
Equipment  calculation,  391 
Erlang,  A.  K.,  403 
Exchange,  C.A.X.,  313 

P.A.X.,  165 
Exchange,  isolated,  313 
Exchanges  in  a  string,  313 

network,  316 

radial  system,  315 

rural,  classes,  313 
Extension  telephone,  153 


Final  selector,  W.  E.  Co.,  237,  240 
Finderswitch^  6 
Finger  springs,  50 

stop,  39 
Fire  alarm  and  police  box,  174 

system,  172 
First  selector,  22 


Frequency  relay,  97 

selector,  93,  257 
Friendly,  H.  M.,  308 
Fundamental  plan,  404 
Fuse  panels,  111 

pilot  lamp,  358 

G 

Generator  alarm,  354 

control  relay,  354 
Governor,  40 
Grinsted,  W.  H.,  403 
Group  busying  relay,  91 

connectors,  98 
Grouping  levels  of  selectors,  135 

sections  of  selectors,  135 
Guide  shaft,  44 


a 


Harmonic  converters,  346 
Hershey,  Harry  E.,  75 
"High  and  low"  bays,  115 
High-low  voltage  alarm,  359 
Holding  time  variation,  400 
Horizontal  chain  relays,  67,  102 
Humidity  control,  357 
Hundred  line  system,  6 

thousand  line  system,  22 


Impulse  springs,  41 

standard  testing,  364 

Impulsing  circuit,  63 

Incoming  connectors,  18 
second  selectors,  19 
selector,  W.  E.  Co.,  240 
trunk  connector,  178 

Individual  trunks,  131 

Information  desk,  373 
trunks,  375 

Intercommunicating  system,  154 

Inter-office  trunking,  18,  22 

Interrupter  relay,  63 
start,  359 
relay,  354 


Johannsen,  Fr.,  207,  402 
Jumpered  terminal  strips,  129 


INDEX 


427 


K 

Keith,  Alexander  E.,  44,  124 
lineswitch,  6 

on  order  wires,  219 
Keyboard,  255 
Keyset  switch,  256 


Large  trunk  groups,  24 
Limited  service  in  P.A.X.,  178 
Line  bank,  54 

connector,  99 

finder,  W.  E.  Co.,  237,  241 

intermediate  distributing  frame,  112 

relay,  57,  62,  70 
Lineswitch,  6 

circuits,  46 

Keith,  44 

rotary,  6,  51 
Line  wipers,  63 
Load  distribution,  401 
Local  connectors,  18 

second  selectors,  19 

secondary  lineswitch  mounting,  112 

secondary  lineswitches,  85 
Lock  lever,  49 

magnet,  49 
Long  distance  automatic  calling,  297 

equipment,  277 
Lorimer  system,  37 
Los  Angeles  system,  409 
Loud  ringing  devices,  168 
Low  voltage  alarm,  323 
Lubberger,  Fritz,  403 


M 


McBerty,  F.  R.,  403 

Main  office  locations,  416 

Major  switch,  6 

Manual  private  branch  exchange,  156 

Master  shaft,  45 

switch,  6,  44,  49 
signal,  353 
Measured  service,  192 
Mercury  arc  rectifier,  355 
Meter,  self-acting,  197 
Midway  office,  150 
Milon,  M.,  403 
Minor  switch,  6,  93 
Miscc'laneous  P.A.X.  services,  177 


Mixed  system,  148 

Molina,  E.  C,  389,  402 

Motor  magnet,  51 

Mounting   lineswitches  and   connectors, 

108 
Multi-office  exchange,  18 
Multiple  arrangement  for  traffic,  390 
harmonic  system,  92 


N 


Negative  trunk,  70 
Night  calls,  P.A.X.,  182,  187 
Non-multiple  harmonic  system,  92 
Non-numerical  switch,  6 
Normal  lines,  5 

wires,  52 
North  Electric  Co.,  Automanual,  253 
Numbering  in  automatic  system,  3 


O 


O'Dell,  G.  F.,  403 

Off  normal  springs,  54,  63 

Office  selector,  W.  E.  Co.,  238,  241 

Offices  more  than  ten,  25 

Open  main,  49 

Operating  expense,  419 

Operator's  aid  system,  207,  212 

circuit,     with    calling    device,    379 
Outgoing  trunk  secondary   lineswitches, 

20,  89 
Overflow  busy  tone,  354 

trunks,  288 


Panel  type  automatic,  236 

Parallel  battery  feed,  79 

Party  line  central  office  equipment,  92 

equipment,  91 
P.A.X.     connected     to    manual     public 
exchange,  185 
to  public  system,  177 

services,  165 
Pay  station,  202 

lines,  280,  289 

local  battery,  magneto,  289 
Plunger,  45 

arm,  45 
Positive  trunk,  70 
Powerboard,  346 

circuits,  349 


428 


INDEX 


/ 


Power  distribution  panel,  349 
plant,  C.A.X.,  320 

for  automatic,  345 
supply  to  selector  boards,  353 
Preliminary  impulse,  134 
Primary  intermediate  distributing  frame, 
112 
lineswitch,  16 
selector,  253 

and  first  selector,  256 
Private  automatic  exchange,  164 
bank,  54 
normal,  49 
Pull-down  coil,  47 

Push-button  calling  device,  W.  E.  Co., 
251 
meter,  195 

Q 

Quad-level  trunks,  81 
Quadruplex  dialing,  303 


i; 


Rapid  fire  toll  serivce,  280 
Receiver,  direct  current,  42,  152 

non-polarized,  42 

series,  42 
Recording  toll  methods,  277 

trunk  circuit,  282 
Relays,  quick  and  slow,  56 
Release  link,  54 

of  connection,  60 

of  selector,  65 

relay,  57,  62,  70 

relay  ground,  103 

signal  alarm  scheme,  360 
relay,  352,  353 

trunk,  48,  63 

when  last  party  hangs  up,  61 
Repeater,  24 

circuits,  70 
Resistance  coil  for  spark,  62 
Retrograde  slip,  125 
Reversing  relay,  71 
Reverting  call  switch,  95 

calls,  95 

calls,  Automanual,  270 

calls  C.A.X.,  324 

sub-office  calls,  413 
Ring  cut  off  relay,  57 


Kinging  conditions,  Autonianual,  269 
current,  346 

distribution,  351,  353 
interrupter,  57,  347 

solenoid  type,  348,  359 
selector,  255 
Rotary  automatic,  W.  E.  Co.,  236 
connectors,  98 
foot  switch,  256 
lineswitch,  6,  51 
mounting,  109 
off  normal  switch,  256 
relay,  57 

selection  of  telephones,  168 
switch,  Automanual,  273 
Rural  automatic  telephones,  309 
line  circuit,  284 

controlling  circuit,  311 
switchboard  incoming  trunk  cir- 
cuit, 286 
Rural  lines,  280 

party  line,  309 


S 


Second  selector,  22 
circuit,  265 
Secondary  distributer,  257 

intermediate  distributing  frame,  112 

lineswitch,  16,  83 
Secret  listening  service,  175 

service  intercom.,  156 
Secretary  service,  166 
Sections  of  switches,  13,  109,  124 
Selective  ringing,  rural  lines,  313 

2-party  for  P.A.X..  168 
Selector  board,  113 

circuit,  62 

duties,  62 

for  reverting  calls,  270 

mounting,  113 

multi-level  group,  67 

section,  partly  equipped,  145 

switch,  62 

10-level,  20-trunk,  65 
Semi-automatic    operation    during    cut- 
over,  339 

sub-office,  231 
Sender  selector,  W.  E.  Co.,  240,  241 
Sending  machine,  255 
Sequence  switch,  panel  system,  247 
Series  relay.  57,  63 

repeater,  79 


INDEX 


429 


Service  desks  changing  for  cut-over,  328 

requirements,  1 
Shaft-operated  contacts,  63 
Shunt  springs,  42 
Shunting  relay,  70 
Siemens  &  Halske  Co.,  35,  51,  227 

traffic  distributer,  222 
Simplex  dialing,  312 

calling,  300 
Single  spool  lineswitch,  45 
Slip  multiple,  124 
Slow  acting  relay,  55 
Smith,  Arthur  Bessey,  308 
Solenoid,  49 
Spieker,  F.,  403 
Stanton,  Leroy  W.,  253 
Starting  relay,  49,  96 
Storage  battery,  345 
Straight  multiple,  125 
Stromberg-Carlson  Mfg.  Co.,  234 

traffic  distributor,  215 
Strowger,  1 

manual  to  automatic,  253 
Sub-office,  30,  72 

in  manual  exchange,  226 

location,  416 

power  equipment,  355 
Subscriber's  station,  151 
Substation  changing  for  cut-over,  327 
Suburban  equipment,  277 

toll  service,  280 
Supervision  by  reversed  battery,  60 
Supervisory  equipment,  345 

ground,  353 

panel,  359 

schemes,  selector  and  connector,  362 
Supports,  terminal,  116 
Switch  adjusting,  365 
Switchboard  apparatus,  43 

jack,  55 

2-motion,  Automanual,  275 
Switching  junctions,  10 

office,  28 

relay,  63 

repeater,  31,  72 

selector  repeater,  72 
Systems  larger  than  1000  lines,  10 


Talking  circuit,  72 
conditions,  60 
automanual,  209 


Tandem  trunking,  28 
Telephone  circuits,  41,  152 
Telephone  Improvement  Co.,  253 

local  battery,  automatic,  310 
Tell-tale  alarm  scheme,  260 

lamps,  357 
Ten  thousand  line  system,  11 
Terminal  assembly,  110 
for  selectors,  116 
symbols,  120 

strips,  116 
Test  connector,  111,  367 

distributer,  367 
Testing  equipment,  345,  364 
Thousand  line  system,  8 
Three-wire  automatic,  38 
Toll  call  checking,  281 

connections  to  P.A.X.,  189 

cord  circuit,  loop  dialing,  285 
sleeve  dialing,  291 

line  circuit,  284 

connecting  methods,  279 
controlling  circuit,  300 

selector  circuit,  291 

service  trunk  circuits,  290 

switching  office,  299 

third  selector  circuits,  292 
Tone  current  distribution,  351 
Toothpick  cut-over,  326 
Traffic,  381 

recording  machine,  382 

variations,  205 
Transmission    selector:  see    "Toll    third 

selector" 
Trunk,  Automanual  to  manual,  271 

calculations,  396 

efficiency,    why   automatic    is   high, 
398 

group  efficiency,  383 

holder.  79 

hunting,  64 

indicator,  W.  E.  CO.,  252 

slip,  124 

symbols,  119 

tone,  63 

dial  tone,  353,  354 
Trunking,  3 

arrangements  and  variations,  107 

Automanual,  254 

devices,  122 

formuhe,  386 

W.  E.  Co.,  238 
Trunks,  two-way,  400 


430 


INDEX 


Twenty  trunk  selector,  24 
Two-spool  lineswitch,  45 
Two-wire  automatic,  38 
Types  of  automatic  offices,  408 


Vertical  bank,  chain  relay,  67,  103 
Vertical  foot  switch,  256 

wiper,  67,  102 
Voltage  regulation,  automatic,  322 


U 
Utility  trunks,  sub-offices,  414 


"Varying"  a  switch,  365 
Vertical  bank,  67,  102 


W 


Watchman  service,  170 
Western  Electric  Co.,  3,  36,  153,  236,  331 
Wiper  closing  relay,  57 
Wire  chief's  desk,  366 
test  circuit,  368 


TK 
63 
S6 
19 


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